I think the longevity point isn't the right way of looking at it - since the II and the AX8 lasted pretty well!
I've had the III since the first shipment into Europe.
Got my FM3 over the weekend
I'm selling the III
Why?
The III is absolutely awesome and I love it, when I first started the FM3 I was disappointed in the power - really easy to run out of power, only two drive blocks etc.
On my III I'm regularly running at 80%+
Then I paused and looked again, I'm going to use an external reverb pedal with the FM3, that pulls down the power a lot - and if I change how I do presets, I don't need 4 drive blocks, 3 different modulations etc many of my III presets have become bloated because of the processing power available - and actually just having a much simpler set-up, with my KoT in front of the FM3 for the boosts (Which works great), I'm good to go.
So the discipline of just looking at what I really need in a patch is great, and I find it stimulating - the FM3 sounds really good.
For the odd sound I think I'll miss the III, but really I don't need dual amps etc. I'm a simple rock and blues guy, and the chance to have a much smaller set-up even at home - is a real benefit to me.
The purity of the thought process with the FM3 really appeals, it's a great gigging tool and that what I really want, rather than 'kitchen sinking' every preset.
My III rig was a III, two EV pedals, a FC6 and a FC12 - sometimes I was adding effects just to use the foot switches!
I'm going to FM3, two EV pedals and a FC6 - and looking forward to it.
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My take - if you need the power the III is an obvious winner, but don't think the FM3 is capable of absolutely everything 98% of people need, especially with one or two external pedals