My commute is 27 miles round trip from Fitchburg to the far SE side of Madison. I do it about once per week during Spring, Summer, and Winter, and about twice per week during Fall.
If the weather is nice, I ride my Trek Soho S (which Trek has since renamed District S), that I usually have set in fixed-gear mode. This bike has been modified with drop bars and a rear rack.
In bad weather, I ride an early '90s inexpensive Trek mtn bike. It's old enough to be from the time before suspension. This bike has rear rack and full fenders. The usual tires are 26X1.75 semi-slicks, but in winter I install a pair of Hakkapeliitta W106 studded tires.
I'm always amazed how I can get to work a lot faster on the Soho fixed-gear than I can on the old 21 speed mtn. bike. It's about 47 minutes vs. 55. The lighter weight and drop bars are part of it, but I'm sure most of the difference is simply 120 psi 700X28 tires vs. 65 psi 26X1.75.