If you're already familiar with FM synthesis then you will love FM8. It has an excellent interface, one of the best. I can't imagine how tedious it must have been to set up all of the routing on a DX7 without the convenient matrix FM8 has! So easy to creat modulation too, and the effects are excellent.
I would also highly recommend the FM8 course on Groove3, it really unlocked the synth for me. I much prefer it's interface over Absynth which seems awkward and clunky by comparison.