The same question has been asked before, but I don't think there's a lot of coherent information floating around about that head.
I have one. I use it for bass, but it sounds great for guitar. All of the things that people say about Sunns being "transparent" and "hi-fi" are totally true; it sounds like whatever guitar you're playing through it. It sounds wonderful (though totally gnarly and distorted) with my bass, and it sounds like a clean, loud tube amp for guitar. With my Telecaster it sounds sort of like the Kinks or something, that kind of sound. It's also really pretty loud.
It's a great head, how much are they asking?