I haven't played with all the assemblies yet, but I have used quite a few (I save-scummed the armoury level quite a few times to check out how common the mods gained were). I'm a cheater I know, but it wasn't so much to cheat as to check out the new stuff. To tell you the truth, I've never played games of DoomRL where mods were so common as now. I've legitimately found 6 firestorm packs over probably 15 games. That's more than I found in 6 months of 0.9.9.1 :).
Note: some assemblies are obviously build and game/challenge dependant. I've noted this where I can. These are only my views, are probably wrong, and only represent my thoughts on the game. Assemblies really aren't required to win as it is, just normal modding will do it. To tell you the truth, un-modded knives and fists will do it, so assemblies are just bonuses. Some let you try new things and some are just plain good. Some let you waste your mod packs or are redundant anyway. This is my list in a vague order of kickarse-ness. Some are about equal on the list, so will be numbered the same. Assemblies requiring whizzkid levels tend to rate lower because they're build dependant.
So here's my review of mods used or wanted to use thus-far:
#1 Tactical Armour and Boots. The pick of the bunch, easily. One or the other are guaranteed in a normal game (chained court mods) and they are incredibly useful. The boots especially, I make them first every time. Both together are lovely. They save space, they regen and they make you run very quickly. Think 0.45 secs a move when running, WITHOUT hellrunner, and even quicker with it. They're VERY good. They make blademaster as good as vampyre for melee runs, fireangel as good (or maybe better) than AotD for shotty runs and they make vampyre absurdly good for anything. Try a fireangel build in a normal game, where a big tactic is just running closer and closer to the enemy, blasting them to pieces with your shotty all the while. Then out comes the missile launcher :)
#2 Speedloader Pistols. Awesome, just awesome. Makes pistols all the better for any build, although MGK or finesse+tech modded pistols may still be better. They just add more options for using pistols as your main weapon with a lot more builds. Great addition to the game that doesn't penalise you for trying something out.
#2 Power armour. I still haven't gotten a nano-mod at the time I could have used this, but it sounds friggin amazing. Slightly better regen'ing Angelic Armour with no WK requirements or anything annoying. It just sounds like great armour all round. Who can't wait ~20 turns between encounters to keep strength 8 armour? You could ditch the rest of your armour inventory and still feel OK (except your tactical armour, always keep the tactical armour).
#3 Stormbolter Pistol. I haven't used it, but I certainly plan to. It's not as though 10mm ammo is exactly rare, and the extra damage would be handy. I actually got two firestorm mods in a AoB game, and so couldn't use them, but I would have absolutely loved to make a pair of these and build for dualgunner. A berserker marine of Armok would be insane fun to play as, just to go all WH40k in DoomRL :). The guns are rare and build dependant (for WK), but they look very, very good. Twin wielded, fire-storm modded, "high-power chaingun" kind of good :D
#3 Assault Cannon. A nice gun, but how nice? The best chaingun assembly is how nice. But it still fires bullets and not plasma, so if you want to not rely on plasma you can use it. It's nice. Especially if you're making a non-ammo-chain (and non-TH) build. Otherwise the mods would be better on a plasma rifle or a host of other things.
#4 Nano-fibre armour. It's good in that it's a reasonably reliable strength 3 onyx armour. It's bad in that it makes you move slowly. And there's usually a few more vital things to do with power mods than debuff your armour. It's probably one of the better options, but I'm getting too used to the speed of tactical armour, and two power modded red armours will probably be better for you in the long run. Good with tough as nails or for AoLT, otherwise I'll be power-modding something else.
#4 Elephant gun. Nice for any shotgun related build or play-style. If you've got shottyman and a couple of power mods then make one. It's a very-long-range, high-spread scouting tool. Nice-ish damage close up, but a DB is probably better. I'd still be tempted to bung one power-mod on the DB, and one on my combat shotty and skip the elephant gun entirely. I guess I'm a bit old fashioned though. Oh, and is it just me, or do shotguns of all sorts have a little more knockback now? My combat shotgun pushes quite a bit of stuff back now, even without a power mod......
#5 Gatling gun. Nice, and lets you try out a bullet led rapid fire run. But there's a guarantee of plasma rifles anyway (and fairly early on in the game usually) along with the ammo to go with them. If you get bulks very early I'd try it, but really a plasma rifle still beats it with damage output AND damage type, and is as accurate with just one agility mod.
#5 Energy Pistol. It deals plasma damage and never misses (assuming you build it the RIGHT way, sniper mod first). If you're a pistol user you'll love it. Especially if you can find a blaster or other unique pistol (or any assembled pistols). Otherwise I can think of other weapons I'd be putting a sniper mod on. If you're building a pistol build, its the second best pistol assembly, so build one as soon as you can.
#6 Plasmatic Shrapnel. I tend to do my shotgun runs as AotD but I am starting to lean towards fireangel. You're almost an AotD soldier with this but still get all the fireangel benefits. As fireangel under AoS this is the mutts nuts. For everything else it's just a bonus. I mean, it's a nano mod. What can't you use it on that makes you superman, with any build? Good, but only useful for AoS runs when you're trying out build ideas, and far too random to rely on.
#7 VBFG9000. The extra splash is good (as it sets off even more secondary explosions) and the regen in ammo is great. But it's one shot per clip. Think of it more like a non-damaging mini-nuke once per level. Start level, fire gun, wipe out everything in area and then be happy in the knowledge that you've got a nice little safe zone. By the next floor it should have regenned enough ammo to rinse and repeat.
#8 Tactical Rocket Launcher. It's a sweet weapon, it really is. But so's a missile launcher. Except a missile launcher does more damage, is more accurate and can still be modded up the wazoo. If you don't get the missile launcher and backpack from The Wall, then make one of these. It's your only semi-reliable option for rockets in a100 games if you don't find a ML early, and bulk packs are pretty common. Otherwise a totally redundant weapon that is outclassed in every field by a guaranteed weapon from a quite nice special level.
#9 Double Chainsaw. You need WK and I very rarely take that on melee runs. It's a good weapon, and it even swings quite quickly with the 2x finesse levels, but you'd have to build for it and hope you get the power mods. The longinus spear is guaranteed later in the game and a bulk-modded chainsaw will see you through up to that point. If you get the mods and want to go slicy on a gunning run, then for sure. Otherwise I guess it's really an a100 or "no-special-levels-allowed" option.
#9 Assault Rifle. I've made this just for a laugh in an ammo-chain run to use up my spare agility mods. It does dish out a good whack of damage with enough levels of triggerhappy, but is it really needed? Maybe in an AoLT without ammo-chain, but other than that I can't see the use. Everything with a level or two of WK and mods dishes out damage anyway, and with triggerhappy being necessary to make it properly powerful you'll probably be going for ammo-chain as it is. And modded ammo-chain plasma rifles rock anyway.........
#10 High power weapon. Nope, don't like it. I can't think of a single weapon that a power or bulk by themselves wouldn't be more useful on. If I need more DPS I'll use a tech-mod or build for it. Maybe good as a backup plasma rifle or BFG, especially if I'm rolling in mods on an ammo-chain run and am just mucking about. But really, you could mod something else up that's more useful, and with WK in ammochain, most guns come out better than this anyway. I guess as a last ditch "I need a better weapon NOW!" option, but it's still kind of cludgy.
#10 Micro Launcher. It takes a perfectly good weapon and makes it a bit crappier. One tech mod makes a rocket launcher better, but two makes it crap. Who knows why? It does have it's good points (fast reload and smaller splash) but it comes with bad points as well(less damage and smaller splash). The bad outweigh the good, by quite a lot in my mind. Some walls (The Vaults spring to mind) also become indestructable to this weapon as well. This, combined with the fact that the rocket launcher is guaranteed in The Arena - which you then take to The Wall and get a guaranteed missile launcher, makes this a totally redundant weapon. The missile launcher out-classes it in every way(and comes with a free Doom-guy(tm) backpack), and a normal launcher is probably better anyway. Especially with just 1 tech/bulk/power or agility mod. Anything other than two tech mods really.......
#Infinity. Really, everything else on the assembly list is just gravy. I don't care if you get the Biggest Fucking Gun. With two levels of WK and all those mods you could mod up a kit that would make you industructable anyway. I'm not saying master assemblies aren't cool, you just don't need them. A sniper, nano or firestorm mod often turns whatever weapon or armour you put it on into end game material without any other mods at all. Master assemblies are like that, but more so.
I really hope we get more basic assemblies. The advanced and master ones are great, but they require at least 2-3 levels of experience to try out these new options. And going for levels in whizzkid has a tendency to draw people towards ammo-chain. Many of the assemblies also lend themselves very well to either triggerhappy or not requiring ammo under chainfire mode. Which makes a common problem in DoomRL (ammo-chain being essentially easy mode) even worse, where you're halfway to the master-trait just so you can try out all the new whizzbang things in 0.9.9.2. I know you don't HAVE to go for MAc, and the new assemblies offer a lot of new options to not go the modded-plasma-rifle route, but it does make a good thing even better. A lot better actually, and it was already the best. Yep, ammo-chain is now undeniably THE BEST, EASIEST, CHEESY BUILD EVER! NO-ONE CAN DENY IT! EVER!!!!!!............
cough.....
My rant is over, I hope you read through all that gibberish. You don't have to agree, they're just my thoughts....