Myself and five others self-taught GCSE Further Maths between the end of January and June of this year. It is certainly challenging and I suggest you only do it if you are actually very good at maths. There are many complicated concepts and unless you are sure you can do it then do not do it. What are your reasons behind wanting to study this qualification? If you're thinking that universities will look particularly favourably upon it then I am afraid you are slightly mistaken. Apart from a maths degree, your admissions tutors will hardly take any notice of it.
Although, if you're capable of achieving the 'super A* (A** / A^) then do it. Trigonometric identities, matrices, the factor theorem and differentiation may prove to be easy for you. Focus on GCSE maths, then this. It should not be a priority.