Yes, further maths is a subject on it's own, like any other, just that it has regular maths as a prerequisite. I'm currently taking both maths and further maths, and coasted through GCSE with no revision (and most of AS, for that matter). Further maths helps regular maths immensely, because a lot of the concepts in AS further maths require things only introduced in A2 regular maths, so by the time you meet it in maths, you'll have covered it and gone well above in further. I'd say if you *are* capable of doing further maths, then taking both is more like an A-level and a half, rather than two full ones. I'm fairly sure, when I get the results from my resit back in Decision 2 (don't know how I managed to do so poorly the first time round, it's ridiculously easy), that I'll be on track for an A* in further maths.
Also I seem to have the opposite opinion to most people, I find mechanics ridiculously easy because it just expands on physics a bit, pure to be my favourite, but middle grounded in difficulty, and statistics to be hardest because it just doesn't interest me.