I've only read her Coldfire Trilogy, but it's one of the best series of books I have ever read. As Darkora mentioned, it is a blend of fantasy and horror, but there is also a very strong sci-fi element to it as well. Very well written, compelling characters (including one of the greatest "villains" of all time, IMO). The interaction between the two main characters, Damian Kilcanon Vryce and Gerald Tarrant is awesome, the magic aspect of the world is well thought out and consistently handled, and the world that she creates is just so creative and amazing that it just cries out for more development. I'm torn that Ms. Friedman doesn't like doing sequels and has long said (and maintained so far) that she is not going to do any more Coldfire novels. I'd love to see more, but at the same time, the trilogy is wonderful and complete in and of itself, and I wouldn't really trust anyone but Ms. Friedman to write any of the characters or use the world.
Definitely give it a try. I've heard that This Alien Shore is a really good novel as well, but I've haven't gotten around to it yet.