I have one (the original and a four button footswitch), it's nowhere near the Axe FX iii in terms of amp models, what is? It doesn't pretend to offer accuracy with any of the modelling, but it's a neat little thing. It also offers various types of synthesis, vocal effects and drum machine emulations. I figure it'd come in pretty handy in a studio when you want something a bit different from the norm.
I can't be trusted re a report on audio quality, I pretty much don't have any hearing above 8KHz, The amps do clean up with a guitars volume drop.
The Mod Duo X isn't released to the general public I don't think although it is out there to early adoptors. Much more processing power but there maybe an upgrade option available to the original Mod Duo owners at some point. The X is more a desktop unit for fx and synth guys.
There are about 250 'plug ins' available for the Mod Duo at this time, some are updated regularly and some aren't. I have yet to purchase any that aren't free cos I don't have the need tbh. The models that do require payment are available to install on a trial basis (silence every once in awhile). I access the webpage graphics (for builing new boards and presets) on both ipad and desktop, both work fine with no issue.
I have to say that I also have positive grids Bias Amp and Bias FX Pro on iPad and both work as well as each other.