Epic
Now, seriosuly, it is a very good game, full of awesine gameplay and graphics, all the Metal gear action you would expect is here, althogh i feel the stealth element has been toned down a bit.
MGS4 is full of epic moments which will probably make you cry if you've been an MGS fan for a long time, and a very elaborated storyline which ends the series just perfectly (Although you won't understand much of it if you haven't played the other Metal gear Solid games).
On it's own, I'd give it 9.0/10. But then there's MGO, which is basically playing Metal Gear Solid online, just as the name implies. the multiplayer is freaking awesome, you'll definitivel have a lot of fun.
Overall, Metal Gear Solid 4 : Guns of the Patriots is the reason why you should buy a PS3.