I wear a mesh jacket with armor when it's hot. The leather vest holds the heat in and gets too hot. The mesh jacket keeps your arms from getting sun-burned and is actually cooler than just a t-shirt. Plus the armor.I have a leather vest, but I actually only wear it when it's cool out.
Also have a Foxcreek Classic Vest. I like to wear it to keep the T shirt from flapping too much in the summer, and works well as a windbreaker in the winter when it's not too cold.
Plus, I continue to deck it out with various pins and patches from the neat places I ride to.