I didn't like JC2. It was boring.
What is so DIFFERENT???
SR2 had a solid story line that compelled you to find out what happened, and very awesome characters. It wasn't exactly realistic however, but in a good way, letting players.. well, richard around as the head of a gang that wanted nothing more than to kick ass and own the city. And that's what they did. The game gives a good story that's worth looking into, while letting the player have some good old fashion car to the streets gun to the face fun. It's extremely good at what it does. It's ridiculous, and it's proud of that. ZeroPunctuation explains it better.
SR3, meanwhile, completely dumps the interesting characters and good storyline, instead favoring to go all out crazy on the "WTF" moments. While still fun in it's own right, it's no where near as awesome as SR2. SR4 looks to just be a huge overall improvement on the series. I can't speak from personal experience because I haven't played it, but SR4 is most CERTAINLY worth the extra wait.