Being both a vocalist and a fan of all things Bemani, I was very (albeit secretly) excited to see this game. For the most part, I wasn't disappointed when I started playing: the game is fun, innovative, the scoring system is surprisingly accurate and the mic adapter works well.
The game does, however, have a score of problems.
1) The included mic is terrible. You can, however, easily scrap a mic from a cordless/cellular phone.
2) The game is very, very unforgiving if you try to sing any of the songs in a different octave. Given that songs for both male and female voices are included, this is more than a little problematic.
3) Just like DDR isn't really dancing, Karaoke Revolution is not really singing. Attempting to sing any of the songs like they're actually recorded will usually result in a bad score. Vibratto will almost always drop your score, and the songs are imperfectly programmed, so you have to pay attention to the visual representation of sustained notes in order to cut off at the appropriate time. None of this should be a problem for the average user, but as a vocalist I was surprised (and not surprised) by the limitations of the game.
But hey, it's a fun game, and a kick at parties (if you can live down the fact that you own it in the first place).