My 'worst thing' was trying to get a hold of one. I intended to be one of the early adopters and populate their available profiles with the select few 'golden' amps I know of here, recorded in a real studio facility. But they cut contact at a point and I wasn't able to get them back on the line to make this a reality, so I moved on and never used the unit.
Try not to get caught up in other people's profiles too much. The ones I've made myself are by far my favorites.
My only gripe is not being able to stack amps on the same performance (single di) without the midrange clouding up and sounding strange even when the recorded tracks are completely in phase.
I hate that there's no kemper vst and I don't think it'll happen, at least anytime soon.
I used the kemper to layer sounds/bi-amp a couple of times, works great!
Never worked worked with real amps for me