I love the idea of Advanced, but I have yet to find an RP in that section I've wanted to join in with, as I'm quite fussy, and pressed for time. Casual is where all of my RP's and the others I've dipped into and out of have been. The ones I run could - by agreement of all of the players - easily be Advanced due to the detail of their background and the average length and depth of posts. 
I've only glanced over free in the past, and I wouldn't want to participate in anything in there - the lack of the same depth and complexity of the RP's puts me off, and I couldn't keep up with the speed-posting either.