I thought it was fantastic... best 7th gen game I've played so far. If you're looking for a mindless shooter, you're definitely gonna be disappointed. I love games that tell and intricate story, though, and MGS 4 delivers that in spades. Plus there is NO other game that mixes combat and stealth in the awesome way the Metal Gear series does.
I would definitely say some enjoyment will be missed if you're unfamiliar with the series, though... if you haven't played them, grab the Metal Gear Essential Collection. If you have the 80GB bundle and thus PS2 compatibility, definitely get it. It's all of like $30, contains the director's cut enhanced versions of all three previous games, and the story will mean a hell of a lot more.
It's like starting the Lord of the Rings movies with Return of the King. You can enjoy it, but you won't get nearly as much out of it as you would have if you'd watched Fellowship and Two Towers first.
Also, try Metal Gear Online. If you like playing shooters online but you're sick to death of all the run-n-gun and grenade spamming of COD 4/Battlefield/what have you, MGO is a massive breath of fresh air.