I own pretty much every Singstar title out there, since I do like singing, and the ABBA game stands out.
Firstly, the track selection is great and all the original videos are in the game. The duet format has been improved by allowing players to choose to sing either the lead track or the backing vocals – which means that classic ABBA harmonies and backing vocals are part of the game as player 1 sings the main song (“Tonight the Super Trouper lights are gonna find me”) while player 2 does the backing vocal (“Super pa, Trooper pa”). It seems like it could be silly, but we were having so much fun we found ourselves dancing along to the songs right away.
To accomodate this the 2nd player's lines are shown at the top of the screen, and often the 2nd player sings at a different pitch to the 1st player (hence the harmonies). As I said, polished.
If you're an ABBA fan, it's a must. If you're a Singstar fan, this is as polished as Singstar's ever been. If you're both, may I suggest you stop reading and start singing?