In regards to the original question, FM8 is awesome! I think FM8 and Massive are two of the best virtual instruments out there, and Battery and Kontakt are two of the best sample-based virtual instruments! The morphing feature and built in effects in FM8 is cool.
There is one downfall for me. While I find many of the presets awesome in the NI instruments, and I do like spending a little time tweaking them so they don't sound like presets, I find that there is almost too much to tweak and I easily get burned out and lose motivation. I'll admit, I'm spoiled. Operator, Analog, etc., have paved the way to peaking my inspiration to create my own sounds. When I work, I like to work fast, and my Axiom 61 controls are mapped perfectly in every aspect with all of Live's plugin instruments, effects, mute, pan, faders, etc..
So in the end, while I like the NI stuff A LOT, I'm now finding that Live's instruments really suit my needs. For example, when creating sounds in Operator, I don't just use Operator, I chain several of Live's plugins with Operator and save the preset, which makes Operator similarly powerful to FM8 (in most aspects).
Accidents are the portal to discovery!