Can't speak for Darklands or AitD, but you should be aware that Descent 1+2 can be a little wonky to control, even with mouse aiming. Blood would probably be my choice. It's a nice, visceral Build shooter with really good gore effects (of course). The level designs are all pretty creative, and there are a lot of them. The combat flip-flops between cheap and frustrating and wonderfully brutal. There's also a load of humor, if you're into the whole "har har it's funny because I understand the reference" sort of thing.

I've played only Blood from these games but I strongly recommend it.
I guess it's the best 2.5D fps out there, it has everything - dark creepy atmosphere, cool protagonist, humorous one-liners, unique weapons, tons of monsters to shoot at and the most important: the best level design in any game without truly 3D environment. Seriously, you got up to 6 keys in each level and as long as you're not tired of looking for them, you'll enjoy every moment playing this game.

Darklands But you do have to read the manual. So buy a different one if you do not like to read. :)

Well, thanks to Ubivis, who is awesome, I no longer have to decide! Thanks everyone for your input.

I'm starting to believe that you and certain other individuals in these forums are made of awesomerine... that rarest uber valuable metal you can find. :P