I currently use an HD500 and overall I'm quite happy w/it. It seems to do pretty well both in a FRFR setup asa well as 4CM. There's really only 2 things that I dislike about the HD500 and Helix someone manged to not address them:
1) The pitch shifter.
It's still monophonic on a flagship product.
As far as I know it also has no concept of formant shifting so sounds pretty 2nd rate/artificial.
2) The 'looper' is very bare bones.
No tempo sync.
No auto start.
These effects feel more comparable to an RP1000 or HD500 than an AX8. While the hardware and routing is brilliant, Helix's DSP feels very 'last gen'. My plan is to give it a year and see what happens and if L6 can't up their game, go w/an AX8. That would be a bummer because I've been using line 6 stuff since the Guitar Port came out and, for the most part, been content.