I commute 24 miles in my work clothes which consists of 5.11 or Under Armour tactical pants and carry a casual shirt and extra under shirt in my bag. If the temps are above 40, I will just wear a light windbreaker with poly t shirt and below that I will wear a Gore Tex Sitka jacket that breathes very well due to all the venting.
I've bought a couple cycling specific jackets, but neither of them breathe nearly as well as the Sitka.
Once the mornings start getting sticky and warm, I'll think about bringing a full change of clothes and showering once I get to work.