I'm a new PS3 owner, and never owned a PS before, never played the MGS series, so, is it good to get the MGS 4 now, without playing the I-III? Anybody tried it?
You should probably check out this thread: http://bbs.clutchfans.net/showthread.php?t=149623 I think if you play MGS4 w/o playing the other games, you'll probably be very confused by the storyline, and you'll also miss a lot of references to the other games, making some things not feel as special as they could. It is a great game though (arguably the best on the PS3...although that might not be debatable), so you would probably enjoy it regardless. If you decide to play MGS4 w/o playing the other games, then at the very least, you should watch the retrospective that Gametrailers did on the series, which does a nice job of summarizing things IMO: http://www.gametrailers.com/game/6748.html
You should get it anyways. I am biased, the MGS series is one of my favorite, along with Street Fighter (I grew up in arcades). Granted, you won't feel the "oohs" and "ahhs" of all the story lines tying together, but the game is just a spectacular experience. Furthermore, the MGS database is soon to be released as a download from PSN. This will help put things together, though it shouldn't be considered a substitute for the rest of the MGS series. I guess another kicker is Metal Gear Online (MGO) which isn't half bad either. You can't go wrong getting the game. You'll probably want to revisit the rest of the games anyways. Just know what you're getting yourself into
If I was confused and I've played them all, you will be out in space. Very fun game, just get ready to watch a lot of movies. IMO the first 3 1/2 acts are awesome, with it kind of puttering out at the end.