I sat Maths and Further Maths with Edexcel and achieved an A in both. With Edexcel at least, the difficulty with Further Maths is that it introduces quite a lot of new topics that you wouldn't have met in just Maths. For example, hyperbolic functions, complex numbers, polar coordinates, and differential equations. If you like Maths, and pure Maths in particular, Further Maths is (in my opinion) definitely worth doing. I found every new topic interesting, and in quite a lot of ways, some of the syllabus of Further Maths helps you to understand some of the syllabus of Maths, or at least reinforces the concepts outlined.
The applied side of Further Maths is essentially just 3 (or 4 if you opt not to put forward either FP2 or FP3) more applied modules of your choice, depending on what you sat for your Maths A Level. I myself sat C1-4, D1 and S1 for Maths, and FP1-3, M1, M2 and D2 for Further Maths, and the only module which I found boring was S1 (which is irrelevant to the Further Maths syllabus anyway).
I found that Further Maths required more thinking about, and more abstract thought. It tends to move away from the tedious formulae and arithmetic that is contained in a lot of the Maths syllabus.
Of course, I can only speak for Edexcel's syllabus, though I can imagine what I've said is essentially true with the other exam boards.