Game play is pretty solid except there's no option to mute the background singing completely due to it being the actual music videos with the original singers playing.
There are 37 songs from the start, and you can download free demos of songs. In addition, there's a SingStore where you can buy more songs, either from Song Packs which are groups of 5 or so songs of a particular artist or group for ~$7.45, or you can get individual songs for $1.29 I think. They have a lot of songs, but doing the math, to get every song would cost over $700 in micro transactions (there are over 720 individual songs you can purchase).
It's also nice that you can adjust the latency value to help with any delays.
Being $19.99 (and $15.99 with GCU), I say you get a fair number of songs. For it to be a $60 game, you can essentially purchase any additional songs that would increase your song list and they're more likely to be songs you want vs songs that you're forced to have.
There's also a Challenges mode where you can accept challenges to try and beat other people online.
Overall, great game for the price.