Well, I don’t have one (yet) myself, but I have been doing a lot of research on them. They’re supposed to be the bomb, man! Supposed to be the best modeling amp out there, in fact. Korg provides the amplifier modeling technology. But, from what I understand, the really big deal is this “Valve Reactor” technology, developed by Vox. It seems that Vox has figured out how to take a single 12 AX7
preamp tube and use it as part of the power amp circuitry to simulate the behavior characteristics of actual tube power amps. Apparently, they are able to simulate the behavior of push-pull or Class A circuits, zero feedback or negative feedback, or whatever they want, using that single 12 AX7. I don't know how they do that, exactly, since I'm not an electrical engineer. But it certainly sounds impressive, especially when you consider that, if they are going to have a push-pull circuit (for example), then the 12 AX7 has to provide
both halves of that push-pull circuit. Again, I don't understand how this is possible, but they have supposedly pulled it off. All I can say is, "Wow!"
The bottom line, though, is that it is supposed to be heads and shoulders above other modeling amplifiers in actually feeling like a real tube amp when it comes to things like breaking up properly when you dig in heavier on the strings or crank the volume knob on your guitar.
Don’t know about simulating a Mesa rig with one. I do seem to recall that it has an “American High Gain” model, which is modeled after a Bogner. I suppose that you would want to start with that if you were trying to go after a Mesa vibe.
But the thing that has me interested in them is that they are supposed to shine especially well on the Fender and the Vox models. (The latter of which would come as no surprise, of course.) I already have a SansAmp PSA-1, which is another one of the better modeling systems out there. I’ve noticed that the SansAmp does extremely well with high gain situations like Marshall and Mesa Boogie (the Marshall models are absolutely AMAZING). But I’ve been less than totally impressed with the Fender and other low gain sounds. I figure if I had both preamps, I’d have just about everything covered.
Anyway, congrats on your new preamp!

I have a feeling you're going to be very pleased with it.