I've tried out a bunch of game demos over the last few days on Xbox Live Marketplace and thought I'd share some feedback and maybe save you some time.
The Good Ones...
Bioshock: Possibly the richest first-person shooter environment I've ever seen, this game takes you to the underwater city of "Rapture," an exquisite art deco masterpiece, where you take on a society of genetically modified citizens in an effort to uncover the mysteries of the city and escape. I couldn't put it down, my dad couldn't put it down... it's amazing and compelling. I found a Barnes & Noble gift certificate I had and just now pre-ordered the Gamestop Limited Edition of this for the additional $10, which comes with a soundtrack CD, behind the scenes DVD, and action figure.
UPDATE 9/5/07: The availability of the limited edition changes so the links to it are flaky. If you want to see more on Bioshock, check out the regular version or try finding one new/used at Amazon.
Overlord: Tired of being the good guy in games? Be the bad guy! This game lets you "construct your evil empire" by ordering your minions about to carry out your nefarious deeds. You run around in third-person as the "overlord" and send your minions to fight, overcome obstacles, etc. I was struck by the humor in this one and had a blast with the demo. Not sure I'll pay a full $60 for it, but if I find it used for cheaper, I'll certainly look twice.
John Woo's Stranglehold: I like John Woo, and while I wasn't overwhelmed by the awesomeness of Hard Boiled, this game was definitely a lot of fun and very stylish. You play Tequila, the Chow Yun-Fat character, a cop trying to track down some cop killers. Running around shooting the bad guys is fun, and the special powers you get and the interactions with the environment make it cool to watch and play. Even Jenn was commenting on how cool it looked as I showed her the demo. Very easy to pick up and play and even comes with a "Casual" difficulty so you can just have fun with it. Again, maybe not $60, but definitely on the list.
The Not-So-Good Ones...
Hexic 2: I played the first one because it came for free with the Xbox 360. This one adds an incomprehensible "battle mode" and removes the easy-to-see colors of the original interface with a washed-out set of colors that makes the whole thing just confusing and hard to play. Deleted.
Fatal Inertia: Spaceship racing with weapons thrown in - hey, never seen that before. Oh, and if you could make it more difficult to control and basically cause me to lose the race if I mess up one turn, that'd be great, too. Deleted.
The Darkness: I collect comics, and I read The Darkness, so I was actually hoping for some serious coolness here. It was sort of neat, but generally I just ran around confused and disoriented, not really knowing where to go or what to do. I got to a point in the demo where I was running around outside some sort of compound and ended up just quitting the demo because I couldn't figure out what to do next. The idea was neat, but the execution... not so much. I think if I saw it for like $20 I'd consider it because when I was able to figure out what to do, it was fun. Can't imagine putting myself through that torture for $60, though.
Track and Field: Message to Konami - please stop porting crappy games to the Xbox Live Arcade. Seriously. Just quit.
Ecco the Dolphin: Didn't this suck when it came out the first time? I, uh, swam around aimlessly for a while, basically not getting it, dying a few times, and called it a day.