As someone who doesn't particularly like anything with Mecha, I couldn't get enough of this relatively short anime. Full Metal Panic (FMP) is one of those top tier programs developed from a great manga that deserves to be way up there along with stuff like Bleach and Naruto.
A fair amount of Japanese humour evades me due to lack of culture knowledge. Sometimes a slapstick joke is deployed in an anime and I don't even understand it, or it just doesn't appeal to a westerner like me. Due to this, I am skeptical about any anime or manga within the comedy genre. However, Full Metal Panic managed to make me laugh and smile throughout, even during some of the crude fanservice that you always have to put up with when commiting yourself to a new anime.
The biggest flaw I can point out is how boring the antagonist is. He has no personality, no background, and is capable of less than a few facial expressions. Other than that, I really can't find much to bitch about. The tension builds nicely and is divided neatly by comedy and romance. Although a little confusing at first, the storyline quickly picks up and you feel like you know the alternate world and the characters within.
I wouldn't recommend this to anyone, you need to be able to put up with or enjoy romance, and be able to watch some mediocre mecha fights. The love-hate relationship between Chidori and Sagara can also be a bit smothering at times, but it's very sweet really. If you want to give it a go, I suggest you watch at least 3 episodes, get a feel for the action and then you'll have an idea of whether you want to finish them all.
After the first two seasons which are mostly serious and run in one long arc, you can treat yourself to the bonus series, FUMOFFU. While it's very episodic and more about comedy than anything else, a vast majority of fans love it and prefer it over the other series. I myself liked all of it and have no preference, but if you've seen this anime or read the manga and have thoughts, please share them with a comment!