Most overated: Call of Duty 4 is the most overrated game on the X360 I can think of at this point. The single player was phenominal and challenging but the multiplayer is nowhere near the quality and balance of another shooter I can think of. (This isn't a fanboy statement because I have equal pun with both CoD4 and H3. A topic for another thread though.)

Most underated: That would have to be Bioshock. By itself it's one hell of a game, it took even me by surprise. Great story, great gameplay and very challenging. Other RPG developers should take some hints from Bioshocks ambition and strive to make games of that quality.

Buying something with achievements is a really lame idea. No company is going to dish out the money to pay for the items we buy with gamerpoints. If you want a better achievement system, create a better discussion thread about achievements and then submit the ideas to the Microsoft feedback forums. Who knows, you might just get what you want. Don't bring up some absurd idea to them or it'll get ignored completely. No one is going to support a crazy idea, it has to be something appealing, rewarding and reasonable.