I took it initially but dropped it around November time, thought it might be interesting to you.
I got an A* at GCSE
Took it because I was intending to take Psychology as one of my AS levels, so I wanted to make up for it by taking another respected subject - hence I was going 5 AS Levels. Turned out there was still space in one of the Physics sets at the beginning of the year so I didn't take Psychology in the end. I carried on with FM until I decided it was a bit unpleasant doing maths every single day, so I dropped it and took Art instead.
I would say if you love Maths it's a great choice, though I thought I really liked Maths after my GCSE years and ended up finding Further lessons insanely boring, mostly because we were doing Decision as part of the course (S1, C1 and C2 in normal maths, D1, D2 and FP1 in Further), but I really enjoyed FP1 for as long as I did it.
I did take FSMQ, and I would say that it probably helps more with the normal maths than with Further. I didn't think any of the concepts in FSMQ applied to FMaths. I thought it was ok difficulty wise but didn't get a brilliant grade because it was taught in a really dodgy way where half our class were taking the exam and half weren't so the teacher didn't focus on it much at all, and I chose to focus on my other subjects.