Wind speed is a lot of fun, one of the guys brought out one of those cool weather stations to the lake bed. After it was all set up and the wind kicked in we started asking all the pilots to guess the wind speed. The answers to this simple question told us why we had so many high wind pilots posting about flying in 40 mph winds. 5mph turned into 15, 10 was over 30, 15 was gale force and 20 was pack it up and get off the lake bed. Well, at about 15 that sand does start to get a bit nasty.
On a lake bed, these portable stations are probably pretty accurate, but if you fly somewhere that has a lot of trees and buildings around, they tend to give you wind speeds a lot lower than you'll get a little above ground level. The hand-held ones are just about worthless. It's fairly easy to memorize the first six levels of the land Beaufort scale, and when you've done that you can estimate wind speeds to within maybe plus-or-minus three MPH pretty easily; I've been doing this for years and checking with the weather stations to confirm.
I agree that 15 mph is about it for comfortable flying. I'd say ten for training. Sort of depends on where you live, though. Some parts of the country get a lot of wind and you'd better get used to it if you want to fly. Others (New York State, e.g.) get lots of almost dead calm days.