I just use it with one persona skin. I barely feel the need to change them - I just use one I like the most for months and its OK. Or at least it was till damn FF updated from 19 to 21 (I checked 20 too) - on both 20 and 21 when I download a file, and downloading pop-up shows (generally something happens with download manager) then personas changes theme/skin to one that I installed like first time when personas were introduced. Now each damn time I download something I need to go to personas icon, change theme to one of last used, and then back to one I'm using and is the only one actually installed. Simply clicking on the one I use doesn't work as its already marked as being used. Well I would need to do all that if I didn't revert back to FF19.
So all in all:
Personas gets a 4/5 - wont get 5th star cuz lots of themes look nice only when you have so much tabs open that they take few lines of space (thus showing more of personas skin image).
Firefox 20/21 gets 1/5 - only 1 star cuz it works, but brakes my immersion, I mean the nice feel of using it - if I'm forced to change skin each time I grab something and download it.