I thought I'd already written a review of PixelExit but apparently not (unless it was one of those "wrote it, got distracted, and flipped over to another tab without clicking submit" that seems to happen more and more frequently as I get older).
In any case, I'm happy to write one now. I'm using the Flat Awesome + style on Xenforo 1.5.21.
Initially, not being very familiar with customizing Xenforo, I had a lot of questions, all of which were answered promptly even on weekends when I wasn't expecting to hear back until Monday.
Since then I've had more questions usually along the lines of "I broke something that was working before: how do I fix it?" - I am an inveterate tweaker - I like to know how things work and "what happens when I do this"?
The styles are excellent. The support is beyond excellent.
Truthfully, I can't imagine going anywhere else for Xenforo styles.