I have the 20W combo. It's amazing, although I am still in the honeymoon period. I've gigged it since though, and it's pretty damn loud. The best improvment over the HT-5 is the clean channel. On the HT-5, it was bland. On this however it sounds sweet. On the 40W and above models they have the voicing switch to switch between Blackstar clean and Fender clean which really does sound decent.
As for the drive channel, I can't stop praising it. It's very high gain though so perhaps not everyone's cup of tea, but with the master cranked as well it could give any Marshall a run for its money.
Guitars: Squier '51, Fender Classic Player's 60s Strat, Epiphone Dot, Ibanez SA260FM (Bareknuckle Nailbomb/Trilogy Suite p'ups)
Pedals: Fender Pt100, Tech21 Double Drive, BYOC Mighty Mouse, EHX Stereo Pulsar, Boss DD-20
Amp: Blackstar HT-20 Studio, Fender Blues Jr