I've just bought a GT-10B and I have to say it's pretty good. I treaded in my GT-6B which i found a bit of a pain in the arse to program, especially in a band situation. Too many menus and too much button pressing for my liking.
The 10B is set out in a much more logical way. It has less knobs but its a whole lot easier to navigate. The EZ Tone is very useful and the screen is big and well laid out. You can dial up your bread and butter sounds in an instant or you can get deeper inside it and program all manner of wacky and wonderful sounds. They've got shut of all that silly auto-slap stuff too which is a plus in my book. In fact all the presets in here are usable.
On the subject of presets. Why do Boss/Roland put bloody preset sounds in these things. Why not do what Korg did on the AX300B and make all the patches writable. I don't need 200 bloody presets.
Anyway, hows it sound?
It sounds wonderful. Not much to complain about here. I even like the OD's. I'm not a metal head so maybe I'm not to judge but they do what I what them to.
I'm not sure if the preamps sound like the amps they are modeling but they are good for shaping the tone and give a nice boost to the sound. I haven't tried recording with it yet mind.
The biggest plus over the GT6 is that FX1 and FX2 now have the same FX list so you have more options to mess with. The A/B FX chain seems like it could be useful but I've yet to figure it out. You can basically switch between two blocks of FXs with the control switch. I'll get back to you on this function.
It also has a basic looper which is great fun for er...playing with yourself.

Don't think you can save them though.

I could be wrong I'm still working it out.
Pros
Sounds great, Its easy to use and its a good tool for jamming with due to the looper.
Cons
Its bloody huge! The LEDs are too bright. You can't see whats written behind them in low light. Sunglasses recommended. Limited looper function and er....no drums

