I don't own one but I have played it a few times.
Firstly it is possibly the best built amp I've ever tried, everything is customisable including a light up front panel that you can have your name put on
The reason I don't own one is it wasn't loud enough for me. Now when you try this amp all you will think is that I am crazy, and you are right. It's not a quiet amp it's just not loud enough for me because I'm putting John Entwistle to shame
Tonally it's very thick and creamy with a fantastic distortion, I have a Mesa Boogie 400+ and I had a Trace Elliot V6 when I tried the GT200. Tonally it's better than both, less hiss, wider range of tone that the V6.
I would really like to own one but at the moment my Mesa's (400+ and Strategy 400+ 900w's of tube power) are what I use to gig, if that need ever changes then GT200 will be my first stop to go on top of my Matamp 8x10 and 4x10.
While you're there ask him about what he said to Gary Moore, that's my fav story.