Of the X360 lineup, I've now finished CoD2, PD0, Kameo, King Kong, and Condemned, and I've been playing Tiger Woods 2006 with a friend on Live. I've enjoyed all of these games, but I'd have to put CoD2 and King Kong at the top of the list with Condemned right below those two. Of course, all of these games were short, so I've actually just started to rent them. Going to decide if I'd be better off with something like Gamefly soon.
As for X-Box titles on the X360, my wife and I are playing Baldur's Gate: Dark Alliance II. I've also picked up Outlaw Golf 2 to play on Live with a friend, and last night I picked up Darkwatch. All of these games run fine in backward compat mode, though BG : DA II does slow down intermittently. Otherwise I've seen no issues. I do wish that they ran in 1080i with widescreen, but I just turn the screen settings down to 480p and use the scaler on my TV to fill the screen...works fine...just not true 16:9 aspect ratio.
Of the games coming up, I'm most looking forward to Oblivion and Far Cry Instincts: Predator. I just finished Morrowind on the PC to prepare for Oblivion, and good lord was it good! Add that depth to an X360 quality visual engine, and I'm going to be lost for months. I also played Far Cry on the PC, but between the enhanced visuals, different plotline with Jack mutating, and Live multiplayer, I'm looking forward to giving it another go.
I gotta say...I really love the X360! I'm looking forward to a much larger library of games, but that'll come, and I've enjoyed all of the launch titles I've played so far...