I've done this for money a couple times, and informally for a few friends. (Plus, a bit more regularly lately as a student at Hacker School, which you ought to check out.)

When I did it for money, both clients were appreciative, but I think it went better with the one I'd previously written a lot of their backend code for, who were also well-heeled. This gave me specific knowledge and more time to give them.

(I could be available for some more of this after the Hacker School term is over, late December.)