Personally, I find it intolerable, have to take gallons of water, and sweat buckets when it gets to about the 22 degree mark.
I love it when it's about 16 degrees - nice and fresh and I don't dehydrate. I can ride all day at that temperature in shorts and t-shirt.
I am absolutely astonished when it's a baking hot day (well into the 20's) and I see a roady powering along wearing a long-sleeve jersey almost completely zipped up over a t-shirt!!! How can you possibly ride like that without dying of heatstroke?!!!