Lumines Live! is great, but a bit pricey (yes yes, it's still cheaper than the retail Playstation Games, but you essentially play the demo, and then pay 1200 Points for another Demo...)
Puzzle Quest is pretty addictive and good, although i prefer a portable Version.
Worms is as great as ever, although it is "just" the core Worms experience without all the crazy weapons and possibilties of the Worms 2 Games (2, Armageddon, World Party)...but within online Play, thats only for the better
Castlevania: Symphony of the Night is considered the greatest Castlevania Game by many, and it is still a very fun and well designed game, with much more content than just about any other XBLA Game.
I also purchased RoboBlitz...wich was "okay"...it looks very good, has cool weapons and good humor, and is overall a nice mix between Puzzler, Shooter and Jump and Run.
Problem is, there is not much story or variety, and although it is sometimes impressive that the game is completely physics-based, you will on occasion get stuck in a situation where the physics do not work the way the developers intended to be.
And i purchased Small Arms, wich is a enjoyable Beat-em-Up/Shooter in the vein of Super Smash Brothers. But unfortunatly, it's punishingly hard, somewhat unbalanced, and only a few people play it online...i like the game, but i cannot possibly recommend it