Hands down, the greatest iteration of the game. I won't break down the points given to graphics, audio, and story etc... simply because they were all exceptional and there will be ton's of reviews for this game all in the A to A+ range. The story takes on a more prominent role in this game to the point that I started falling asleep through a couple 15-20 minutes sequences. These can be skipped but they flesh out the story the first time though and are presented so fluidly and interchangebly with gameplay that you can't fault them too much for them being so long (longer so than any of the earlier games, or any game for that matter).
Aside from the long cutscenes they have pretty much set a new standard for graphical quality on nex gen systems. The water effects, explosions, facial features, and environments are all top notch. The graphics are so good that it becomes a bit awkward when the camera ends up taking a high def close -up of Snake's crack (some things are better viewed at low res).
I have experienced 4 boss battles so far and there are numerous ways to defeat each. On Solid (Normal) mode the bosses seemed a bit easy but I have no doubt that since there are 2-3 difficulty levels higher there should be a challenge for every skill level. Boss fights fit the classic form of either taking them out lethally or by non-lethal means. This provides different items depending on which route you take to dispatch them and also adds to the replay value.
The sound effects and musical score are also classic MGS style but taken to the next level particularly if you have a good surround sound setup. Kojima has implemented a new feature in this game which produces a circle around Snake during gameplay to symbolize his sense of the environment. It is awesome to see how well this feature matches up with the surround sound system. It's hard to explain but when the circle indicates an enemy approaching from the left of the character for instance you can hear the footsteps right where they should be coming from. You have to experience this one to understand how well the two systems work.
The multiplayer seems pretty standard with the exception of the tedious registration process. I have only tried the team deathmatch and it was fun and at a quicker pace than the game so I didn't fare so well the first time through. Other than that the graphics still look crisp and there are an assortment of features to create a unique character as well as weapon set-up.
In conclusion, if you have a PS3 then you must own this game. Even if you don't like the series you should get this game just to see what the PS3 is actually capable of. If you consider MGS 4 the benchmark for MGS games on the PS3 I can only imagine what the games will look like 4 years down the line. Consider the leap from Sons of Liberty to Snake Eater and the anticipation is simply killing me.