I'm a very lucky owner of an HD500X and I'm especially impressed by the amps. For me, everything is better than my old combination of X3 PRO with the Behringer FCB 1010 (Uno) MIDI foot controller. I think its very easy to get the sound I want and just plug in and play on gigs. As with most stuff nothing is perfect and some of my thoughts are:
I'm a Linux user and as a programmer, I have really big difficulties to understand why they - as of 2010's - have not made an editor that also runs on Linux. I do not believe for a second that they do not how to make the editor run on Linux as well, so I guess that it's a strategic question. More and more companies are taking Linux users seriously and a relative small company as Harrison Consoles says that around 14% of their Mixbus DAW customers are Linux users. That is not an awful high %, but Line 6 is not a tiny company, so even a Linux base at 5% should make a considerable high number of users.
The use of % as parameters for so many FX parameters is strange, but I use my ears until I'm happy. However: I can understand that this is big issue for many people.
The lack of mono output on the XLR side and no internal power supply. This is in fact bad. I have to use a DI box on one line out to the mixer/PA. Imagine how much better it looks on stage without that spaghetti.
I think it's strange that they did not add a really clean (JC) amp as standard in stead of one or two amps that is in marginally use, but I will be one of the first to buy amp packs when the 2.6 FW comes out.
Despite the issues above, the price and what you get back ratio is hard to beat and as a working tool on stage and as an inspiring joy provider when I'm playing, it's unbeatable! I have so much fun when playing with the HD500X.