I commute, and 23-48F seems to be ideal. I'll ride down to 10F, but then my hands and feet get a little cold. I'll ride up to 100, but have to take it easy.

The hardest part of riding in the cold for me is waiting to hit the 10-15 minute mark where I really warm up. I "under-dress" for that, so I am a little chilly until then, except leaving the house or office, where I am still "over-dressed" for room temperature. If I am delayed getting outside I can over heat and start to perspire which is not good when hitting the cold outside air.