Great gameplay overshadows the bogged down (but good) plot, lacking cinematics and mediocre voice acting.
Sons of Liberty, released before Twin Snakes (and the game which Twin Snakes' gameplay was based on), felt at first like pretty much the same game. The main difference, of course, is that after an hour or so you aren't Solid Snake, but a new hero named Raiden. Though disappointing at first, as Raiden isn't the most badass (nor well-acted) character, I didn't mind how Raiden played, nor his overall persona (even though he's rather weak-minded). However, as the game progressed, I found myself more and more disappointed in character development. Though Raiden and Rose and their relationship are all developed well, things aren't always that believable, and both characters just really aren't that interesting. Most other characters are barely developed at all, and those that are lack much interesting characteristics (particularly the enemies; the main boss is mildly interesting, but all others are completely boring). Snake still plays a big part in the game though, and he's still the best character around.
Overall cinematic presentation, unlike the first game, was very lacking. Most cutscenes start, show something minor, then revert to codec conversations. It gets really, really repetitive, particularly when the game goes into it's many, many pointless ramblings. Though usually there's a point to them, it's often minor and could've been explained much more explicitly. I also dislike the game's use of live action video used when explaining things; it just doesn't fit well. I also wish some little things were better explained and flushed out. Voice acting could've been a lot better too, particularly that of Raiden, and in the emotional scenes (no one knows how to cry on this cast, and I can't help but wish they just wouldn't have).
Despite the many excessive ramblings, explanations or backstories which bog down the plot extremely, the overall message is there and gets across effectively. I couldn't help but get that satisfied feeling after the game's final cutscene. And, in true Metal Gear Solid fashion, the plot is given closure yet left open with a few unanswered questions at the very end. It's worth the playthrough, though I doubt I'll ever go through it again.
What really makes MGS2 though is it's intense, stealth-based gameplay, as well as several action sequences that are very well done. One sniper portion of the game was really entertaining, all the boss battles were unique and enjoyable, and playing with Raiden's sword at the end of the game was great. Hiding in boxes, corners, over ledges, in lockers or other areas is funner than ever and always satisfying, thanks to the game's great level design and solid play mechanics. My only complaint in this area is the game's controls; they get a little clunky sometimes, and it always seems to be in times of desperation, where an enemy is just about to get you in his vision and you just can't pull off the quick hide. You get used to them, though, and can make due easily anyways.
Overall, if you like any kind of stealth action game, you have to play MGS2. Even though the plot could've been cleaned up a lot and characters could've been more interesting, you'll be satisfied with what Sons of Liberty has to offer.