Anyone who knows me, knows that I'm not a huge fan of d20. But I must say this is a great class!
The first thing that jumps out at me is the wonderful character art at the beginning, and the stylish and attractive background. It's very well done and really makes the document fun even before I get to reading it. And once I started reading it, it only got better. This class is presented in a way that gives enough information to start my imagination churning, but doesn't go so far as to provide too much information to squash the awesome ideas it spawns in my brain. While some parts are pretty technical, I find that's just an element of the game it's made for.
One of the things that has me most excited about the Pyromancer, is the end goal. I'm a big fan of classes that present a big change when you reach level 20. Instead of being just "better", the character actually undergoes a change and becomes something more than he was before. I really think this class would be a lot of fun to play.