I just got Noxious Gas, and I'm a Necro MM. I thought it'd be great since my zombies generally melee, and had been looking forward to it a lot. I was very very disappointed. My lowest tier guys, which can be relied on to run into battle, also often get one- or two-hit KOed so I don't like to use it too much on them. My second tiers, which shine in melee, like to be stupid and stand back using ranged attacks when I want them to run into battle so I usually have to waste time forcing them up. My main complaint about the ability is the duration is so SHORT while recharge is pretty long. Its basically a waste to cast it before battle starts, because it barely lasts a portion of the battle before it wears off, and then you have to wait another 4 battles or more for it to recharge (if you're powering along without resting, which I tend to do as a MM.) You also have to to babysit who you cast it on and try to maneauver them to have the greatest effect, since the radius doesn't seem all that big. I have yet to see anyone vomit from it. I usually can't seem to affect more than one or two guys and since most of the effects are status its really hard to tell if its doing much at all. I'm going to give it the benefit of some more solo testing and slots, but so far I'm sorely disappointed. Pretty useless in teams since your members tend to scatter and kill the mobs so fast. Oh and you cant' slot it for Accuracy, only Recharge, End cost, and something else I forget but I'd never put in it. If it doesn't get any better I'm definately respecing out of it, I wish I'd taken Neurotoxic Breath and still have plans to get it, but I havn't had room yet since I took Leadership and Stamina. The graphics for casting NG are also nothing special, its about the same tossing of a poision thing as everything else, and your minion gets a little green gassy cloud around him.
I plan on respecing now that I've had time to test-drive some abilities and see what works best for me since I have a couple things I'd rather trade-in for something I'd use more. 
. I skipped Envenom for other things. Also do most people not put many slots in Soul Extraction? I love my ghost, I actually gave him 2 extra damage and 1 holding, and I find with 2 recharge reduction SOs he's almost ready to go again when his time is up so long as he hasn't died. He's also stellar for pulling. I'm not a number cruncher so maybe he's not that great even enhanced, but he serves my purposes well.