Before the HT20 I had a Vox Valvetronix and was never happy with any of the distortion models. I tried to bypass the awful distortion with a Zoom G9 multi fx but, if anything, that sounded even more synthetic
The Blackstar has a nice buttery overdrive sound of its own with really silly amounts of gain on tap, but I am astounded by how good the Zoom amp models sound through it, that piece of kit has really come to life now.
I can't really tell you how this unit compares to other valve amps because its the first one I've had, but I did a fair bit of looking around and there isnt much else (if anything) in this price range with 2 channels and an effects loop.
Finally, I would just like to mention the volume levels because I guess some of you will be torn between the 5w and 20w models like I was. I'm finding the 20w one is fine for practicing in the evenings at home with the master and channel volumes set real low. You really couldn't crank it at home though, it would damage your hearing.
Its overkill for a practice amp but I think its great ;-)