I'd personally only be wearing it when under 10 degrees but it depends how hot/cold you run. I tend to go for a long sleeve jersey and the Brevet Insulated Gilet (best bit of kit ever) in anything warmer.
IAmNScoot, but the Brevet gilet has an internal taffeta liner which the Pro Team doesn't have. Makes it more comfortable and gives an teeny bit of extra insulation.
It keeps you warmer when you're not moving, which is good if you're like me and get perilously cold the second you stop riding. It stops me from shivering like a whippet at coffee stops. I also like the hi-viz stripes.
I'd personally only be wearing it when under 10 degrees but it depends how hot/cold you run. I tend to go for a long sleeve jersey and the Brevet Insulated Gilet (best bit of kit ever) in anything warmer.