I do not use a Mac although I wish I did, they are price prohibitive for me right now.
However I have used Celtx for windows and I was equally unimpressed because of the missin g support for RTF.
I use Page to stage which was on it's way to dethroning Final Draft then they stopped developing it. Still it provides almost the same features as Celtx as well as the ability to save as RTF and import RTF, it's freeware but there is no Mac version.
Celtx: Screenwriting For the Rest of Us