I've been playing the PS3 version lately on rental. It wasn't bad, the storyline was fairly decent and the cutscenes were often very good, sometimes quite amusing. The gameplay was nothing special though; the missions were perhaps somewhat more varied than GTA4, but not by a huge amount. The vast majority of the time you'd just run in guns blazing, whereas GTA4 at least encouraged different approaches at times.
There are a lot of extra activities and missions, some of which were quite good. The environment is quite interesting too, there's a lot more buildings which you can enter and there's some quite interesting and cool stuff to see.
The problem I felt was that the basic mechanics of the game just feel...rubbish. The vehicle handling was utterly abysmal; all the vehicles seemed to handle fairly similarly, in a generic, arcade-style manner which just feels wrong in a game such as this, especially for one so modern. I had absolutely no desire whatsoever to just drive around, explore the map, try different vehicles and find those that I really liked; difficult to choose one when they're no different from each other. I've played games where driving isn't the main focus of the gameplay that have better vehicle physics than SR2.
The radio stations also feel more like they're just playing in the background rather than being part of the actual game, it's as if they just don't quite 'fit in'. I'm no fan of the GTA4 radio stations either, but they do at least feel and sound right.
I completed it yesterday, partly because the storyline, missions and cutscenes were decent, and partly just for the sake of having something to do. The fact that I sent it back today says a lot though; I just had no desire to keep with it after those good parts were finished with.