I own a Kemper and pre-ordered the Helix a couple of weeks ago. I've already lined up a buyer for the Kemper.
I certainly wasn't unhappy with the way the Kemper sounds (provided the right profile is loaded), and in fact I'll be amazed if the Helix sounds anywhere near as good. But I was getting frustrated with its flexibility, or lack thereof, particularly where floorboard integration, MIDI I/O, and routing were concerned. Enter the Helix - where Line 6 seem to have been specifically focusing on all of the things that were driving me up the wall in Kemper-land.
If the Helix sounds even half-decent, I will be in heaven with this new UI and all of the routing options. And I won't have to spend all my time worrying about which footcontroller to use, how to program it, wire it, etc.
No knock on the Kemper - if you're in the studio and want uncompromising tones, or you're on stage and don't mind a more complex set up, the KPA is a great solution. But I need something simpler, and hope the Helix delivers.
Fingers crossed that the amps will be on par with e.g. Postive Grid's Bias Amps, but with lower latency.