Well, I never went further than 6x lvls, but it's real fun to get from 30~40 to 60. I also loved the 2 class change (Shadow and 2nd class change) cuz that was real fun
On the other side, it may be true that higher lvls aren't as fun (if u understand fun as lvling and getting new gears/skills) but that's obvious, every single MMO it's like that, so... no much to say about that.