Beat the game twice, here are some pros and cons. No spoilers!
Pros:
Amazing graphics! Regardless of what you have seen in the trailers this game will surprise you visually. All of the cutscenes are done in real time and are not pre-rendered. On top of that you can zoom in during cutscenes as if Kojima himself is daring you to find a flaw with the graphics, which after two times through the game I must say I haven't. Furthermore, things like explosions, weather, water, wind, infared, and many other elements have been done to a level of high finish. This is what next gen is all about!
The story is something that I enjoyed as well. I haven't been so attached to the main character of a game in years, and I have to say that the game does an excellent job to put us in snake's shoes. The cutscenes are a bit long but they really help develop the characters throughout the game so I am not sure if making them shorter would be wise or unwise. After all, it is the cutscenes that really help you connect with what's going on. Plus, there may be room open for a MGS5 so what's not to like (Unfortunately I doubt that Solid Snake would be in MGS5 though). Sometimes the story is a tad bit confusing but near the end of the game everything gets cleared up in case you missed something earlier.
Weapons Customization is also a really cool feature. You can add laser sites, dot scopes, regular scopes, shotgun attachments, grenade attachments, a flashlight, and grips. On top of this you can buy emotional ammunition that makes enemies cry, laugh, etc, as well as buy different colored smoke bombs. Also with Drebin's shop you won't be out of luck when you need to buy ammo as long as you've been doing a good job of picking up weapons from people.
Because of these weapons options, you can now truly decide to run and gun through the game or use stealth. Unlike past MGS titles you can now buy guns and ammo any time you want to, so as long as you are killing/knocking out enemies and taking their weapons you shouldn't have to worry about running out of bullets. Plus, on larger maps the enemy will continue to respawn over and over until you decide to move further into the level (this doesn't happen on smaller areas). The first time I played through the game I was really aggressive but the second time through I found it just as fun to make sure I made it through the game without having the "alert" phase happen. So, two totally different ways to play are possible, plus 5 different ways to play with the different difficulties, and yes, the Big Boss difficult is incredibly hard.
The sound is also a very powerful element to the game with an excellent score by Harry Gregson-Williams but also the background noise, gun sounds, and character dialog are done to near perfection. I highly recommend playing with headphones if possible.
Cons:
Sometimes the cutscenes do feel like they are dragging on, but with past Metal Gear games I am somewhat used to it. I didn't experience any 90 minute cut scenes but there are some that are right around the 60 minute mark, but at this point in the game it's somewhat understandable why it's so long. Overall I was not too disappointed by this, especially since you can drive around the little MK II robot in the mission briefings (these are some of the long cutscenes at the beginning of each act).
The single player experience feels a tad bit short (technically speaking you can beat the game in under 5 hours if you run quickly through every level). The flip side to this is that replaying the game over and over to unlock things does not feel like a huge hassle and the game doesn't overstay it's welcome. For me, I felt like Oblivion and even GTA IV have a tendency to feel like they are never ending so with MGS4 it doesn't feel like this huge chore to say "ok i'm going to play the game over again but this time I will do X and X differently".
Conclusion:
I've beat the game twice so far, and wanted to make sure I played through it at least twice before writing this review. If you are a MGS fan purchasing MGS4 is a no brainer. If you don't have a PS3 but want the game bad enough in my opinion it is worth it. The game answers a lot of questions, and the newly released MGS database helps out even more, especially for new comers to the series. On top of that the gameplay is the most fun it has ever been, there's tons of stuff to do and unlock, the graphics will impress even those that don't like video games, and for true fans of the series I have to say this game is incredibly special. As it has been said in many game reviews, Solid Snake's story comes to a fitting end.
This is one of the best games I have ever played in my 17+ years of gaming, I highly recommend that you don't pass it up.