I find AV to be a fun Religion, and I'd call it my most used. As others have stated, Sacrifice the Weak is a very awesome Civic. 1 Food per Pop lets you grow some really sizey cities, especially if you can keep up with the happy cap
(or in the case of the Calabim, don't care since you're going to eat them). You also get +10% coin, and while losing out on some GPP hurts a little, I find the health bonus to not be a big deal.
As has also been mentioned, the Infernal Grimoire wonder can nab you a free tech. Usually around the time I build it I use it to pop one of those super-expensive techs
(around the 8K beaker mark, like Pass Through the Ether), but even earlier you have plenty of pricey and useful techs that you can pick up for free out of it. Never really found myself using it as a worthwhile piece of equipment but you also have the option of that.
As mentioned earlier, AV Priests rock. Ring of Flames is awesome, provided you keep your units inside the stack of doom
or well away from the flames. However, I can't say I've ever found their UU, Diseased Corpses, of any use
(although I've seen others mention curing their disease and using them like Champions - without disease, 6 Str + Metal Proms at 90 Hammers is nice if you can handle the micromanagement of disease curing them).
As the others have said, they have two pretty good heroes. Rosier can go running around as a lone skirmisher quite comfortably. If you ever pick up Orthus' Axe, feel free to fork it over to him and let him rack up some easy kills as he picks off random weak folk
(whenever I use Rosier I tend to get him Mob I+II and Commando and let him play like a lone Hippus raider). Can't say I've got around to using Madero all that much though but he looks pretty solid to me.
As for hell terrain... well, that all depends. I usually play Small Maps, so the AC is slightly skewed in my favour. I can imagine the AC being quite the pain on Large/Huge maps where you have more Civs to contend with. I usually find that I don't get to spread the Veil around all that much
(especially compared to RoK, which I can have half the world worshipping easily through trade-routes alone), so that doesn't rack up the AC. Even if it does spread about, you could war with your own evil kind. Evil cities and AV cities getting raised brings the AC down, doesn't it?
(I'm sure it does but someone correct me if I'm wrong)
Avoid the Prophecy of Ragnarok if you aren't the Sheaim, Infernal or a Barb-friendly civ, since that'll help the AC on its way up. I wouldn't vote against building the Stigmata on the Unborn though. Free beakers, free Entropy Mana and a little Stigmata promo for all the units born there is worth the 5 AC point trade off in my opinion.
If Hell terrain becomes a
real problem, then I suppose you always have the option of switching religions. RoK (or Emp) will turn you neutral, which'll slow down Hell terrain from entering your borders (HT won't enter neutral lands until AC 75). A religion switch hurts a bit but so long as you haven't built your strategy too much around the civic/heroes, you can get by, provided you've got the religion firmly spread about to keep the pop happy. The Priests stay too, which is an added bonus and means you won't put too big a dent in your military.
All in all... is the AV worth it? In my mind, hell yes!