Coming from someone who's never played any other MGS....
Based on what I've read about the entire MGS story, it feels like alot of the game caters to the hardcore fans that know the plot well and have played the previous ones. It also seems that "I have nano machines" is the standard response to answer somethin they can't explain.
The game starts to feel abit slow once you get to Shadow Moses. I personally thought fighting those massive hordes of mini gekko (I forgot the names) was abit boring. Things got really awesome once you start driving REX, but I was really dissapointed at how long it lasted. Mechs are pretty common in games nowadays, but MGS4 is one of the few games that makes it feel sooo good - animation, sound, etc, it was great.
Once that's done, the action seems to wear down. The battle with Screaming Mantis was silly. Reminds me of the Leviathan battle in Resistance 2 - more for looks rather than actual gameplay. A little after that, they do the split gameplay/cutscene thing again, except it's alot better this time around, since you can actually watch it since you're only tapping X the whole time. The music and cutscene were amazing.
Overall I think it was a great game, but has an insane amount of cutscenes and definitely not enough gameplay. The installs are abit of an annoyance too - sometimes I load completely different chapters just to do some of the cool parts of the game, and having to go through 3 installs kinda sucks when you just wanna play the game.
I bought it for $35 used. I don't really having any regrets about it. I beat the game about 2-3 times before I started only slightly bored of it.