Socom 2 was the best online shooter for the Playstation 2 system in it's time, but it has been outshined by Socom 3/CA.
The single player portion of the game isn't as good as the online simply because it is slower paced and not as challenging. (although it is the hardest of all the Socoms in my opinion.) And the ai can be a bit brain-dead at some points. This can be overlooked however, because if you have an internet connection, you will probably forget this game even has a single player campaign!
Overall at this point I suggest you get the newest socom. Socom: Combined Assault. But when Socom 2 was new, it was the best there was and I loved it.