Well, first thing to say is that if you didn´t watch the 1st season you won´t understand a lot, so...
This is undoubtelly the BEST anime I´ve ever watched, not only the story (which is not very very rich itself) but because of the fun you´ll get with it, Full Metal Panic is really AWSOME in that sense ! Sometimes you will cry of laugh with it, I recommend it to all thos who like school stories and a bit of fight ^^