I find that anything over 15mph tends to flip my planes over on the starting bench. That's usually about my limit if I am planning on whether or not to go to the field on a given day. I always base my flying days by what it says on www.usairnet.com for my closest airport.

If I am at the field and the wind picks up, I'll pretty much fly no matter what. The only real hard part is landing. I'll leave my floatier planes in the car and fly one of my land-on-a-dime planes, or one of my don't-really-care-about planes.