I have just bought the HD500. And to put it into context, I have owned the original POD bean, a PodXT, a VOX Tonelab, a Zoom G9 and a Digitech GSP1101 in the past, as well as owning and building a variety of classic tube amps and clones, including AC30, JTM45, Marshall 1987, Marshall 2204, etc. So, anyway I think I have a half decent ear.
I'll say that the HD500 is quite easily the best modeller I've owned. Especially for all those slightly breaking up and lower gain tones, which are absolutely cracking.
The cab sims are much better than the XT's and in fact all the other modellers I've tried. Still not awesome, but pretty good, and really very varied. If you can't find a cab sim to work on the HD then you're not trying. Ultimately I'd rather mic a cab, but for silent recording I'm still impressed.
The stomp box models are also a huge improvement on any of the other units I have owned.
I have never played an AxeFX or an eleven rack.
I will also say that, when paired up with the Line6 DT25, the HD500 is utterly, utterly phenominal.
So completely believable in it's dynamics and everything. I played it side by side with a new Vox Ac15C1 the other day and the Line6 absolutely nailed the same tone.