Brand new Fractal user. Coming from Atomic Amplifire 12. I really did like that unit and its functionality. I was a bit worried about the long term prospects of the company so when the FM3 came around I took my cue to make the jump. I couldn't be happier so far, but admittedly have not played any gigs yet. Full disclosure, I am a middle-aged hack that plays in a middle-aged hack cover band.
The FM3 edit is a breeze to navigate and make changes to presets. The three foot switches, despite their massive flexibility, is not as user friendly as 12 footswitches. No way around it. That said, I think I have managed the presets, scenes and effects such that it should be workable in a live setting. The reality is that for 90%+ of the songs we play I'll need one amp sound and maybe a bump for a lead. Changing effects intra-song is pretty rare so I do not think it will be a problem over time. It's possible I'll look into an additional stand in switch at some point if it proves necessary. I don't see me going for the FC6 as it won't really be worth the cost. At least for me.
There are a ton of different options to manage your sound on this unit. Between presets, scenes, channels, control switches, etc., I think you would be hard pressed to find something lacking. I suppose if you require kitchen sink presets, massive ambient swirling tones, or whatever else you might imagine, you may bump into limitations; but, for most general purpose playing I think this unit is hard to beat. And I haven't even managed that the sounds are terrific.
Get one when you can and don't look back.