My first post ever here on the Fractal forums was about this very question. I got flamed a little, about "this has been talked about to death", but there were some really helpful people (especially M@tt) that helped honestly answer my questions, so I'll try to do the same.
I've owned the Kemper, AxeFX II, and AX8. The Kemper had some great amp tones and great "feel". With the constant firmware updates, Fractal products have caught up in that regard, IMO.
And while, in contrast to most other people, I struggle with Fractal's effects (especially the delays, coming from a TC G-System, which has the best effects I've ever heard in a multi-unit, particularly the delays), they far blow the Kemper's out of the water. EDIT: a bit more about the Fractal's effects: I believe that the Fractal is capable of delivering effects close to (or, in some cases, equal to) the G-System. But they are a lot harder to dial in. The G-System, I could just hit up the basic parameters and it all sounded amazing. I've gotten some amazing effects on the Fractal but it took a LOT more dialing in. Even then, I still haven't quite been able to mimic some of the most useful effects from the G-System, even though the Fractal probably has a lot more options and a broader range of parameters. I don't typically use the more out there extremes of any effect. But the basics are harder to get on the Fractal units, IMO. Either way, the Kemper's effects have been improved over the last couple of years, but are still a joke in comparison, and don't come anywhere close.
The Fractal units are also FAR more flexible...you can pretty much tweak everything if you want, and it all sounds good and works like "real" amps. The Kemper, as soon as you start tweaking away from the core built-in sounds, loses fidelity and feel.
If you're comparing the Axe II or III with the Kemper, the above alone is helpful. When you throw the AX8 into the mix, you also have to factor in portability. For me, the AX8 has become my main gigging rig and I've traveled all over with it. It goes FOH and sounds awesome.
For practice/home use, I have a CLR Neo that sounds amazing, but you could get away with pretty much any FRFR powered speaker with it.