My ride yesterday was in fairly brisk winds from an approaching storm. I don't mind headwinds that much. But the memorable part is when riding downwind just at the speed that matches the airspeed. It's like floating in a hot air balloon. Nice.

Oh, and as for the original question...somewhere in the mid-to-high 20s, I wouldn't ride for fun. I would for commuting to work, though. Work is work, and fun is fun.