I got this game the other night and had a quick play on my own when I got it. I struggled to get a decent score on the dancing part, and the singing part was a bit easy to get a good score.
I agree with the comments here about the dancing being nothing related to what you are doing with the controller, which is a shame. But, the biggest disappointment for me is the songs, where Singstar on the PS2 has the original artists performing, the songs on this game are dodgy cover versions with bits of lyrics missing. The speed of the songs is a bit off too, and the way it scores you is not as in-depth as Singstar.
I had more fun with it tonight, but that's probably because I was playing it with other people and alcohol was involved.
Overall, I would say, its good enough fun for a party, but it's just not Singstar.