Bought this on launch, and was dissappointed to find that one of the microphones was partly broken - no lights during the songs curiously.
The negatives for the game are plenty unfortunately, and my comparison here is SingStar, a game which I've bought 6 volumes for on the PS2. As I've moved to the 360 I was really hoping this was a SingStar beater, so that I don't have to buy a PS3.
Anyway, the negatives: (1) the game often slows down during songs, as if it's having to process information. Which seems bizarre for a karaoke game. (2) You can't login singers under multiple Xbox Live accounts, so all accrued points and rank changes go under the logged in Live account. Which just makes a nonsense of the system, particularly the ranking. (3) Although there are the versus modes, they aren't particularly exciting, and certainly don't compare with the simple versus modes on SingStar. I don't understand how this could have been missed, as it's part of what makes SingStar so great. (4) One of the biggest problems is the game is far too easy. You can sing pretty much off pitch and it gets registered as a correct note. Similarly, for rapping you can sing a constant note and get full points! (5) The instrument shakers are a new idea, but they don't add to the game. (6) Similarly the movements are just annoying, and get in the way of singing.
A handful of postives about the game: (1) The lights on the microphone are cute, for a bit (2) wireless mics are great (3) there's a reasonable song choice, in my opinion (4) if you don't want a discerning karaoke game, this is the one for you.
In summary: better to buy SingStar, if you have a PS2/3.