"Mixed reactions", and "been there, done that", BUT...
On-line play is a completely different world. You can play co-op with 1, 2, or 3 of your friends from across the continent. It makes the game much more fun and interesting, especially if you and your buddies playing co-op take it somewhat serious. That's how I ended up actually beating the game: playing the co-op on-line with some of my GS friends from the SKS. To make things a little more interesting, even after you beat the main story line, you can go back and try to earn "badges", like accuracy, medic, grenade kills, etc. So, right now I'm working on getting all the badges. I think I have 21 out of 34 or something. As far as playing the "regular" on-line run and gun, that's a ball too, if you can find the right room to get in. Too many people cheat, glitch, and spawn-camp, but, that's to be expected, yet unfortunate. Some of the on-line games you can play are games such as Demo: Plant a bomb at the other teams base. Suppression: Kill everyone in sight. Escort: Escort a friendly out of the area, and so on. Sound: Pretty good on a surround sound system. My favorite line: "mmmm spicy!" Music gets repetative after a while, but it's not too irritating.
Graphics: Decent for the PS2, I've seen better. Looks much better with the right tv and connections.
Replay/Gameplay: Well, that depends. I play (on-line) at least twice a week for hours on end. Sometimes longer. Last night for example, I played co-op with a friend of mine through the first 6 missions of the game. We were having a blast and you can really get seriously into it. Value: If you're a fan of the other Socom games, it worth adding to your collection.
Overall, a pretty decent game. It has it's flaws, but you can totally get past the dumb "AI" if you wish.