Saints Row 2's superior combat and flat out more enjoyable gameplay give it the edge over GTA4.

User Rating: 9 | Saints Row 2 (Collector's Edition) X360
The first Saints Row had a problem with reviewers being afraid to give it the score it deserved - as in, "I don't care how good it is, since it's a GTA-style game, we can't score it highly or people will mock us for praising a clone!" This wasn't the case with all media outlets, but it was still quite rampant. The same seems to have happened here. Despite the "8"s you are seeing from reviewers, this game is easily on par with GTA4 and in some was surpasses it. And don't let anyone try to fool you otherwise.

SR2 doesn't reinvent itself, but it is damn fun. The combat was already vastly superior to the GTA series with the first title, so not much needed to change this time around. Character customization is also another great feature that you can get lost in for quite a while.

The overall attitude is more light-hearted/less-serious than GTA, and it only adds to the enjoyment of the game. These titles are asinine to begin with, it doesn't help when the storyline is ultra-serious.

If you have the first game but didn't think too much of it, you won't necessarily be wowed with the change from the first game to the second. However, if you LIKED the first game, there is zero reason not to pick this one up.