MGS2 is one of the first PS2 games, and one of the best.
MGS2 literally takes the Stealth genre and makes it its own. Back in 02 this game was the best of its genre, now it has been bested by some other games, and even its sequel. But this game still holds up even by todays standards.
The Gameplay is astounding, fluid, and put at a pace so you can calculate what to do and where to go. The gameplay switches sometimes throughout the game, for example there is a section where you are escorting someone throught a flooded part of an area, but the gameplay usually stays the same for the most part. The controls are sometimes a bit fiddly. Especially when you want to lean, peek and shoot around a corner, you are going to have to hold 3 buttons to do that. You get used to it in time, so it isn't that big of a deal.
The boss battles are great. They are thought out well, and quite a change of pace from all the sneaking. Each boss battle is different, and instead of just gunning him/her down you actually have to think out their pattern and exploit it.
The story is one of the best I've ever had the chance to take part in. Full of shocking twists, lovable characters and excellent action sequences, you are surely going to love this story. Sure, the cut scenes can go on for a bit long, but it is all adding to the story and/or explaining key things.
This game is one everyone should play. If you haven't played the previous MGS, there is a written article devoted to catching players up. A must buy.