Its a niche area I'd say but if you get it right it can be really fun, I personally spent hours on one for months before it was eventually shutdown.
In my experience, things that make it playable and fun are things like, leader boards, and personal bests: the idea of competing against friends for best time makes them pretty awesome to ensure that you are the king of parkour. Another thing I have liked on these servers is the range of different parkour's, multiple themes, maybe even co-op parkour's.