I used Realistic Leveling for a long time, always with excellent results. Then I changed to nGDC and, to be honest, something didn't quite work for me. Maybe was just too used to RL or maybe I wasn't specializing my character enough, but it wasn't as fun for me as RL is.
(Note that most of the time I don't like to fiddle with the technical details of a mod - I don't like to configure a mod to suit my tastes, my approach is 'install and forget about it'. I'm sure that if I had configured nGCD to work as I wanted, it would probably do what RL does - but then, why would I do it if RL already does it?)
So I'm back to RL and I'm happy.
I've used Oblivion XP in the past and even though it is an astounding mod, superbly done, I still prefer the traditional Elder Scrolls leveling system.