Rocket Riot (review coming soon), is probably one of the best Xbox LIVE games on Windows Phone 7 right now: catchy music, great combo of retro and 3D graphics, fun gameplay. Only downside was cost: $6.99 made it one of the most expensive games on the Marketplace, which even I avoided paying.
Luckily in the last few days it dropped to a more reasonable $4.99, making it more worthy of consideration. Only thing we're hesitant about are the few complaints about the game "not loading" after a few days of playing. This seems to be isolated but hopefully the developer will have a v1.1 soon.