I think it's great!
A "standard" MIDI footswitch will work. The MIDI CC charts are published in the manual.
You can also use traditional push button switches - there are two 1/4" switch inputs on the back for footswitches or expression pedals.
Really depends on the profile the Kemper is playing. I would not say "fizz" is an inherent problem in the Kemper.
There are almost 2300 user profiles posted on the Kemper site. Some of them are crap. Some of them are quite good. It takes some time to hunt them down.
There are also commercial profiles available for $. Some people hate that. Those who buy them seem to think the time vs $ tradeoff is worth it and the commercial stuff, especially theampfactory.com, gets high marks.
I play mine live once a month with a QSC K10. I've owned the HD500, I still own the 11R and the Kemper. I can't make informed comments on the Axe FX box.
My opinion is that the Kemper, overall, sounds better than the 11R and the 11R sounds better than the HD500. That's my take.
Both the 11R and the Kemper, in my opinion, have better user interfaces than the HD500.
Here's where I go into stream of consciousness mode.
I have no idea why I'm writing this much. Maybe I'm lonely tonight.
The 11R's approach to having the amp models respond much like the real amps respond to particular knob settings is really cool. The Kemper does not do anything like that. The Kemper EQs are more generic and not interactive. The Kemper's Gain works better when lowering it than increasing it. The general consensus is you can reduce the gain of a profile by about 20% before it sounds "weird", but you can only increase the gain about 5% before it sounds "weird/bad". I agree with that.
I find the 11R's interface easier overall. I like how many knobs are on the Kemper and things are well thought out.
The Kemper does NOT have any kind of computer editor - and with 2300 profiles online, it can be a headache to share patches because there are just so many.
HOWEVER, all you need to share a patch is a simple USB stick and a computer. You can freely download/upload patches to their website without having a USB dongle hanging off your computer or face DAW incompatibilities with your computer just to manage your patches (like the 11R).
In this regard (patch sharing), I think the HD500 has the best online software (free - standalone). 11R's computer editor is great but a good number of people hate... strong word... yes hate that they have to have Pro Tools installed just to get to it. Other people are fine with that part.
The Kemper development is alive and appears to be thriving. Some features are still missing that were expected to be there from launch date - like Performance Mode. Initially the beta release of Kemper has some pretty unstable software. That has improved greatly since OS 1.5x. The HDxxx series is still getting updates. The 11R is in limbo and commonly thought to be "dead". No one knows for sure.
There has been public complaints that the Kemper suffers from "digital aliasing". The general consensus, and I am generalizing, is that the aliasing can be heard only in extremely high distorted cases playing very high on the guitar. I've heard it. It occurs very rarely for me and it's generally on patches that I wouldn't use anyway. The developer has acknowledged it, has suggested it would be an easy fix, but would be fixed at the expense of future DSP expandability. No resolution yet. Just being up front on that. Some people are more sensitive than others.
The Kemper is considerably more expensive that anything Line6 or the 11R. You could buy a HD500 and two NEW 11Rs for the cost of a Kemper, or you could buy an HD500 with one of Line6's tube amp things. Don't forget the overall cost of a monitor solution and foot controller solution. It adds up really quick whatever you buy.
It does NOT come with DAW software like the 11R. It CAN NOT be used as a computer interface like the 11R.
For live work - I'm not a professional - I played the 11R and the Kemper through the same QSC K10 in the same environment (practice room). I find the Kemper to sound more natural for me. To me, the 11R has some IRs that has a certain high frequency quality to them. The Kemper can do that too, but it's very profile dependent instead of permeating the device as a whole. I couldn't make the HD500 sound good - possibly my fault - I'm not interested in learning "tricks" to make patches sound good or fight DSP limits. Although I thought my HD500 sounded like crap through headphones and better through my K10.
Many people feel the Kemper excels at "raw amp tones". The FX are generally considered to be high quality, but with fewer choices than the AXE FX. (I am summarizing what I've read on forums for you - these are generalizations).
Hope that helps.