Definitely a masterpiece. I only wish that there was a bit more gameplay.
This game is fantastic. Every gamer needs to experience it.
When I first heard that this game has over 9 hours of cutscenes, I was worried that it'd be more movie than game. Instead, Kojima ends up producing the perfect blend of gaming and cutscenes, creating an entirely new gaming experience. This combination of cutscene and gameplay is perfected in the final boss battle. That battle, I must say, is the THE most awesome final boss fight I have ever played through. I don't want to spoil the treat you're in for by describing it. You just have to play it to know what I mean. It's really something else, trust me.
The only thing that would have made this game better is if they had added more gameplay to the second half of the game (starting from the 3rd act). If you remove the cutscenes, those 3 acts are quite short. Still, there are many things that add replayability to the game, which more than makes up for it by making MGS4 fun to play again and again. I won't get into into the details anymore because much has been written about the extras that make MGS4 worth playing over again.
And then of course, there's MGS online, which should definitely add a lot of replay value, though honestly I can't say much about it because I haven't played it. I can't access the PlayStation Network from where I live (bummer).
Other than that complaint, this game is practically perfect. It's so good that I'm actually contemplating on buying a limited edition MGS4 gunmetal PS3 that I found online.
MGS4 is, most definitely something every hardcore PS3 gamer should own. If you don't own a PS3, go get yourself one, or play it on a friend's PS3 (which is what I did).