I have one and it is really great at what it does--replicating the sounds of recorded tube amps.
It's important to realize the difference between listening to a recorded/profiled amp versus hearing one live in the room.
When listening to a recording of a Kemper vs. a recording of a tube amp, they are virtually identical. Playing one live is different though and you have to get that before you will really appreciate it.
I started out using mine through a small PA and while it sounded "good", it was lacking for me. So I now run it through a Fryette Power Station and then into my 2 x 12 guitar cab. Gets much closer to the "in room" sound to me, though still not there 100%. And of course I lose some of the authentic sounds of the original profile by playing it through a power amp and guitar cab.
There are a lot of great profiles out there, but I have used commercial ones exclusively from M. Britt, TopJimi, Soundside, and The Amp Factory. Many of those sound outstanding and it is fun to play through "amps" that you would never ordinarily get a chance to hear.
I don't play out so I can't tell you about playing one live but there are a lot of people who do it successfully with a monitor for the stage and the main outs direct to PA.
If you have any specific question, let me know. I've only had mine for a few months and there are numerous tweaks that I haven't taken the time to fiddle with yet.
I also had a custom cabinet made for it which works great. Here's a pic: