Played Duke 3D on school PCs back in the 90s, bought it (on XBLA) back in 2008, but I didn't get serious about playing through the whole game until this spring. I just finished the moon base levels and now I'm back on Earth fighting through a sushi bar. Can't really say enough how fun this game is. I'd go as far as to say that Duke 3D is a more entertaining single player FPS than anything this generation with the exception of Valve's Orange Box games. The levels are so well designed with flying (jetpack), swimming, lots of secret areas, and some limited environment destruction (like a cave-in or earthquakes). Love the weapons too, from the standard shotgun/rocket launcher to more creative weapons like the freeze ray, shrinker, and pipe bombs. Still haven't figured out how to effectively use the laser trip-wires though. Oh and I love the rewind feature. Sure it takes some challenge and tension away, but it's nice to be able to go back to any point in a level and try again. Wish more games had that.
Anyway, I saw that this thread was bumped and I wanted to throw in my 2 cents. At $5 this game is a steal. Tons of content, and much more creative than most modern FPS games. Just be sure to try the demo first and make sure that the game doesn't give you motion sickness. It's not using polygons and the walls warp around a bit if you turn while look up or down. That plus the fast movement and 60fps refresh does take some getting used to.