The STORY of Full Metal Panic! is very interesting. A man named Sousuke - who's part of an anti-terrorist group called Mithril - must go to a high school to protect a young high school student named Chidori due to her being one of the "whispered". That is, she possesses knowledge of an advanced technology called "black technology" that everyone is trying to get their hands on. This could have been a fantastic plot had it been presented better, but the anime did a poor job.
The ANIMATION for Full Metal Panic! really isn't anything to brag about. It looked very 1990's to me and, for me personally, I don't like it when anime looks like that unless they were actually made in the 1990's. Otherwise it just looks sloppy. I didn't really like any of the mecha designs either.
The OP for Full Metal Panic! was fine, but nothing exciting and the voice acting wasn't great for me. I watched this English dub and couldn't stand it, but I continued on because I was watching it with a friend - I wish I had seen it in Japanese. For me, none of the voice actors did a good job except for Sousuke. And they mispronounced Chidori's name the entire time, which is annoying.
The CHARACTERS, once again, were just a bit below average. Sousuke was interesting with his socially inept ways of dealing with things and Tessa was an interesting choice for captain - I liked that she was only like 15 or 16 and was a captain who was a bit inept herself. That made for an interesting character. However, all of the others felt like they were taking away from the show rather than adding to it.
My PERSONAL ENJOYMENT for Full Metal Panic! might be more of a 4. I did not enjoy this series. Normally I like mecha anime, but this just had so much promise that it let slip down the drain that I couldn't bring myself to appreciate anything except for the concept storyline.
OVERALL, I probably would not recommend this anime unless I knew that the person had read the light novels or were just so into mecha anime that they want to see all of them.