I'm fighting the Peacewalker (I got to the "to be continued" and haven't tried the battle, yet) and so far, there hasn't been one single moment where I've said to myself, "this is so awesome". I admit, I haven't tried mult-player but then I don't know anybody else with a copy of the game.
MGS had the Psycho Mantis, Cybor Ninja, Metal Gear, etc.
MGS2 had the tanker, sniper area, swimming section, harrier battle, etc.
MGS 3 had the Boss, the End, Shagohod, Ocelot moments, etc.
MGS4 had the motorcycle chase, Gear vs Gear battle, and the microwave scene, etc.
On top of that, all of the games had characters that actually had personality, tragedy, and in general, made you care. What does Peacewalker have to make me care? The story is sorta weak (unless something huge is going to happen soon), the characters are bland and forgetable, and the boss fights have either been: Defeat the platoon, defeat the machine, or defeat the platoon escorting the machine.
I hate to say it but.......this game is getting the sales it deserves.