I played it for a couple months and really liked it. Well, Team Sneaking and Stealth Deathmatch, at least--everything else was too action-shooter-y for me. The stealth games, of course, were ruined by camping and blitzing, though, which happened about 85% of the time. (In Team Sneaking, it especially sucks because your stealth goes off when a teammate gets caught). And of course, there are the high-level douches who you *wish* you could make eat their words but, in all honesty, they truly are better then you.
What's funny is that after a long break of not playing, I came back to find the same level-obsessed players going on about how they used to be a "level 16 FoxHound" but de-leveled or something. If anything ruined MGO it was the level system... and the lag... and the players... and the glitches... among other things.
I have to say, though, playing as Raiden and Vamp is always fun. It made me really look forward to MGS Rising.