I never played any Metal Gear games. But damn, I wish I did.
You really should. Quite easily some of the finest games in the video game industry. If you like a good compelling story with food for thought that can translate into real life and tons of philosophy that will rattle your brain, with fun and quirky gameplay and hilarious moments that don't take themselves too seriously Metal Gear is for you. The series as a whole is amazing and a true gem among the piles of mediocre games out there.
MGS4 is possibly the best entry point for newcomers, though there will be a lot you won't understand as far as the story goes. MGS4 is also the worst game in the series as far as story goes, but the best when it comes to game play.
Story:
MGS2
MGS1
MGS3
MGS PW
MG1&2
MGS4
Gameplay:
MGS4
MGS PW
MGS3
MGS2
MGS1
MG1&2
Ground Zeroes and Rising look amazing as well. I'm hyped for Rising.
Also, the themes and philosophy are what really sold the series for me. That ending in MGS2 was BRILLIANT. Say what you want but there has never been a game series as enthralling as MGS.
That whole deal with Meme, and what creates ideas. God, it was SOOOOO GOOD.
Here's an analysis someone did on it,
http://junkerhq.net/MGS2/Analysis.html
if you're into anime like Ghost in the Shell, or movies like The Matrix, MGS is right up your alley.
I don't normally say this about video games, but Metal Gear Solid 2 IS ART.