I'll throw in my 2 cents.
Viva Pinata = outstanding game. Sold well over 1 million. It is 'different' but appeals to all age groups. I have friends that are die hard Gears warriors, and yet love Viva Pinata. This price is a steal.
Kameo = outstanding launch game. Platformer style, controls are different, but you will be amazed with the quality of this launch game. Again, for this price especially, a very good game, but totally different than mainstream (like Viva pinata).
PDZ = based on todays standards, not that great a shooter. At launch, and for the next many months till Gears 1 came out, it was one of the most played games on the box. Multiplayer was really fun. That being said, 3 years latter, there are just too many better games out there, so I would not recommend.
Prey = I have not played yet, but I am tempted. I'll read up a little mor about it.