Do people ever use them, and how useful on average are they?
I sorta came over from Crawl - a much longer and substantially harder game. Teleports there are a risk, too, but the level maps are much bigger than DoomRL's maps; big enough that the chance of being 'ported into a worse spot from a bad spot is acceptably low. Not super-low, but low enough that it's generally safer than trying to tough out the current situation. It also helps that in Crawl, good healing is not usually available in big enough quantities to allow damage soaking for any serious length of time; you *need* ways to fix your enemy woes reliably and/or quickly.
DoomRL sort of gets this reversed; full healing is carried around easy enough, armor soaks damage as well and you're generally either carrying the most reliable means of foe death available or you're already pushing up daisies. Maps are much smaller, meaning you've got a better chance of running into bad guys. Totally fits Doom, sure, but the risks of random porting tend to far outweigh the rewards as a result; it's not likely that you'll do better to use a phase device than to heal or even to reload. The problem is that because of that the phase device is left by the side of the road when something better comes along; full kits, armor, even ammo and mods, which spurs one of two questions: why is an item that is significantly less valuable than all others in this game, or is there a way to make this more worthwhile to the average player?