AS Maths is split into Core Maths 1, Core Maths 2 and one other module. FSMQ covers about 90% of Core Maths 1, 40% of Core Maths 2 and small bits of "one other module".
It's a very good qualification in my opinion - well structured and all. Very heavy focus on algebra, by the way.
What's the point in doing it? Not much, really. If you get an A, it gives you 20 UCAS points, which is something you can tell universities about. But most of the better ones wouldn't care so much about the 20 points. It was sort of something to kill the time with between GCSE and AS for people who did their GCSE a year early at our school. It used to be an elitist qualification, but it was decided that it was too difficult, so it got dumbed down some years ago. It does help with AS maths a lot though.