Dealing damage equal to merely two times the hp drained is definitely not worth it. Taking a look at killcounts, in a typical win I probably deal over 10,000 damage (after armor!) while taking perhaps 1000 hp in damage. Preserving your health is fundamental to surviving, so if you're trading it away for something, that something better be really good.
That's probably why I don't think a health draining gun would be appropriate as a main weapon, unless it drained at a very low rate (1 hp per 10 shots?). Instead, it'd be a viable option if the player was in a situation and thought to him/herself: "Well, if I were to fight this out the usual way, I'd probably lose x hp, but if I used to health drain weapon, I might handle the situation well and only lose y hp to the health drain." So I think a strong, fast single-target gun (to use against viles or other dangerous targets that could lead to major trouble), or a pretty strong area effect gun (to use against dangerous crowds that can really deal out the pain) would be useful archtypes for this weapon.
However, currently there already are weapons that fill these roles - whatever build you use, the matching weapon will be the fast strong single target attack, and the BFG9000 is the weapon to use against crowds. So this necrogun will have to be better, to offset its downside.
I do think it's a good idea for a unique weapon, though.