Hey! I could get the UAC logo!
Been playing a little Doom 3, August?
Naw, not yet. But I've spent countless (don't even try to count 'em) hours playing Doom II and all it's permutations, add-ons, levels, WADs, MegaWads, et al.
have you played the Marathon games? Besides M1, M2, and Minfinity, there are countless robust add-on scenarios.
plus, as opposed to DoomII, it has a delightful story. see
http://marathon.bungie.org/story/ for backstory, faqs, spoilers, and reams and reams of speculation as to what is really going on in the game.
and, just so's ya know, the engine linked in the first link is OpenSourced and allowed by bungie, and the 1st game's scenario is fully avaliable and allowed by bungie. you can buy the 2nd & third, and use the opensource engine with their scenarios, if'n you liked the first enough.
just keep in mind it's from 94 or whatnot. you can't jump without using a grenade launcher, etc.
but, you CAN see all the way around an enemy, as opposed to them swivelling as you go around them, etc, etc.