I always watch. I'm fascinated by it. if I were younger, I'd be inclined to take it up, or at least try it out. I think gymnastics training is a must. After that, I think you'd need extremely high confidence in your shoes; one slip could mess you up good.
As far as the activity itself goes, to me it feels like almost a sport of sorts, except I hate "sports" that are scored by judges and I don't know how else parkour could be scored. Other than that, it would be a great way to run from the cops.