I was half dragged into watching this series, but I must say I was pleasantly surprised by what I saw. Much as the great title suggests, the series is mech based. It revolves around a special force called special "Mithril", and an elite specialist soldier named Sagara. Add to the mix a schoolgirl named Chidori and you've got the title - Full Metal Panic!
The mechs used in the show are far from the standard fare. They're unique enough and the Mithril force is unique enough that it manages to feel fresh. Sagara is a dead clone of Heero Yui, but that's not a bad thing. Sagara is described as the most "professional of the professional", an elite soldier through and through. Taking this pure military mind and placing him into a urban highschool situation is actually quite funny. There's some hilarious moments that alone make the series worth watching.
Chidori bugs me a bit, but she serves her purpose in the series. I just find her about impossible to like early in the show. I'm quite sure this is purposely done though, and as the series goes on she becomes much more likable.
The action sequences is where the show really shines. Elite forces, big mechs, guns, explosions, tanks... lots of fun for all. It's very well animated and very well timed. Being from GONZO, I would certainly expect this. They've done every bit as good as they did with the Vandread series.
If you like the title, you'll probably like the anime. It's really as simple as that. A Heero like main character, plenty of fan service, GONZO's wonderful artists, and a storyline that makes you want to watch just one more episode every single time. It's a refreshingly well put together show.