I'll chime in. I have a Mustang III with fizzing. I've read everything (well, a lot) on the web about it.
The Good: the amp, minus fizz, has a very natural, organic, tubey feel and sound to it. Very enjoyable and responsive. (I'm not a real tube amp guru; I'm only really interested in lower gain Fender and Vox tones, and I've only owned and worked on a few Fender tube amps.)
The Bad: If your amp fizzes, forget it for *any* kind of clean playing, and you traditional dark-tone-loving jazzers should run from it. Play a note on your low E string, and hear a strange, raspy "eeeeeeeooouuuuueeeeeeeooouuu" overlayed on it, and... go look for another amp.
The Maybe (is that a category?): I hear it's only the Mustang III and up that do it, but that they can develop it over time. Mine has. If you can live without the power, fine. Get a I or II. But if you're gigging, do you want to live without the 4 button foot switch to switch effects individually and to provide a handy floor tuner? The 4 button is only for the III and up. Are you OK with needing a computer there so you can tweak your settings? If not, then you, like me, don't see the Mustang I and II, mic'd or not as such good gigging amps.
As for me, I'm selling my III. Some rocker will like it. Vox, Tech21 or someone else will get my money now. I won't even try a Mustang II now. Not because Fender won't *fix* my amp -- I have low expectations as to them admitting a problem that might result in a lot of people returning cheap amps -- but because they won't even *tell* us what the problem is so we can fix it ourselves.
I'm sure they know exactly what's going on with these amps. All they need to do is say "Hey Mustang III owners, we're glad you love your amps, and for your modders who don't mind voiding your warranty, here's something to try that might make you like your clean tones even more!"
But they won't even do that, and it pisses me off, as you have probably noticed.

So goodbye Fender.