Here is a link to a thread I posted on this exact same thing: Friesans in dressage?
. Perhaps you'll find some useful information there.
The general consensus was that Friesians are not built for the dressage work as really they are bred to be cart horses. There are the occasional ones who do 'make it' but they aren't really suited to upper level riding if that is your goal. I've also been told by a clinician that I've ridden with many many times (who has gone to the Olympics, and rides and trains multiple Friesians regularly) that Friesians are really awful to ride. I'm sure they aren't all though.
As for jumping, I'm not sure. I would assume they could do it at a lower level but likely wouldn't have the scope for big jumps.
The crosses with the warmbloods seem to have some success. I've seen a really great mare come through the dressage rings and was very successful (although I don't recall her competing past 3rd? Level). I think it really all depends on your goals. If you want a scopey upper level jumper or dressage horse you're probably better off to find something bred for the job. If you're looking to just have fun and aren't looking to do upper level work then maybe a Friesian is your kind of horse!