I've had experience with Friesians, and I own a Friesian Sport Horse. He's half Friesian, half Paint.
Friesians are wonderful horses. Someone on the thread mentioned that the ones they worked with were hot heads. That tends to be the exception rather than the rule. Most Friesians are laid back and easy to work with.
Here is my only beef with purebred Friesian--they tend to have more than their share of health problems. Many Friesian people believe this is due to the extensive inbreeding that was necessary to keep the breed alive. So far, the studbooks have remained firm in their refusal to allow the infusion of outside blood to improve the breed's constitution. The breeder I bought my sport horse from has lost four of eight Friesians over the past five years. And I'm always hearing about someone's Friesian dying.
That being said, the Friesian crosses seem to do very well healthwise and in competition. Purebred Friesians are sometimes good dressage horses, but it depends on what their body type is. I wouldn't recommend them for jumping in competition--they're just not built for it. The crosses, depending on what their breeding is, can be very athletic. They often retain many Friesian characteristics such as a calm temperament and slighty heavier build. They also tend to be somewhat cheaper than a purebred :) Good luck in your search!