The Virtual Amps are available as standalone and VST/AU plugins (one installer for Mac and Windows). The amps also have an Impulse Response Player for cabinet simulation and WAV recording capability in the stand-alone software.
In my option the sound is on par with all digital SW and HW amp simulations currently on the market (even better for my ears regarding HF digital artifacts, especially with head phones).
The Virtual Amps are free to test them out in demo mode (short interruption via dialog box after a couple of minutes).
More info and sound samples, under:
If you like the sound of the Virtual Amps and send them a short audio clip (~1 min) performed with one amp models in DEMO MODE, useable as sound example/promotion they give you the amp license for free (until end of April).
Email: email@example.com (mailto:firstname.lastname@example.org)