I use my amp with 3 guitars: a Fender Strat, a Gibson Les Paul, and a Jackson DK2T. I've plugged into a few different cabs and speakers - more than I will list here. All of the guitars sound great, but the thing that made this amp open up for me was to use it with the Celestion G12M Greenbacks.
There is no type of music out there that this amp can't cover. The master volume on it is amazing, so unlike many amps, you can get amazing tones at any volume level - though of course it sounds better louder! The distortion is creamy and organic, thick but not muddy, and can break through a mix with easy while leaving enough ground for others to be heard.
One of the best things about this amp is the way it plays. I find myself able to pull things off with it that would be nearly impossible on my other amps - a '99 Triple Recto and an '83 JCM 800 2203. It's extremely responsive to your playing style and how hard you hit the strings, so the feel you're trying to get across when you're playing really shines through with it!
I had a problem with the amp that turned out to be an impedance mismatch. Mesa Boogie makes their cabs available to use in a mono or stereo configuration, but the connections on the jacks they use aren't insulated. I bought a used Recto cab a little while back, and as soon as I did, my XTC went down. Still worked, but the bias resistors for the power tubes were overheating and de-soldering themselves. As I've insinuated, the connections on the cab had come together, shorting and changing the impedance to 2 ohms when it should have been 8 ohms.
The reason I mention this is because before I knew it was the cab, I was putting my Marshall and Mesa Triple Recto into it with no issues. All things considered though, I'd much rather a resister get de-soldered than an output transformer need replacing. Other than that, I've had zero issues with the XTC. Fantastic build quality and VERY good layout on the board!
There is a downside to this amp: it's difficult to set up. There are so many factors to how you can do it that it can be overwhelming, and though it's easy to get a good tone, getting a tone that will blow you away may not come for a bit, or at all with the wrong speakers. Point in case is that I'm used to having the presence control at about 11:00 on my JCM 800 and completely off on my Recto. With the Bogner, I run it at about 3:00!
Now that I've owned it for a while, I'd say there's nothing out there that can come close to what this amp can do. There may be other amps that can take on the same styles with different approaches that sound just as outstanding - Diezel, some Mesa gear, older Marshalls - but the Ecstasy is truly a unique amp.