It entirely depends on the mare and stallion used, there are alot of both breeds which have quite weak backends and long backs especially the modern breeding fashions of both breeds, so crossing the 2 would give you something heavy with no engine! I have seen the odd nice one - there is one on youtube that jousts but if you look at his conformational shot he's got a weak bum!!
I guess it goes for crossing any breed - the 2 animals used must compliment each other, if you specifically wanted a dressage horse with friesian in it then I would go for the TB/Arab/Spanish/warmblood crosses as you will get something slightly lighter but still heaps of bone but with more stamina and agility. Also crossing 2 breeds that are bred to use working trot most of the time is probably not going to be as great at canter/gallop as they are much more suited to trot - again though a generalisation and there are plenty of individuals of both breeds that are good at the other gaits.
Also depends on wether you are heading for classical or modern dressage, if classical then go for a spanish cross(warlander) as they tend to be compact and perfect for high school work - after all andalusians are what gave friesians the trot and hair!!, if modern dressage then a TB/Warmblood/Arab cross would be better suited.
Here is a video of one of my TB cross babies - sorry not the greatest video but gives you and idea of the action - plus he's going to be a big gg - he's just over 15.3Hh and is 2 next month!!
Also still a really gawky youngster and his bum has just put another growth spurt on and we hogged him as he looked like a hippy!!!!
http://www.youtube.com/watch?v=ELiE9gJFQAE
What we are aiming at with adding the TB blood is to cross back to a friesian so you will get similar results to the Arabos as you get the added stamina and agility but with the friesian looks and temperament. We may also be looking at doing the andi cross as again it is going back to how the friesian breed was started but outcrossing will help get rid of some of the many genetic problems the friesians have due to their tiny gene pool, but selecting the right mare to cross with is a nightmare - all the suitable ones we like with just the right action and conformation are £15000+ - eeeek!! - better start saving!!!