I've commuted solely by bike a few times over the last 20 years or so, usually on Bike-to-Work Day. The round-trip is just shy of 100 miles, so it's not sustainable.
Most of the times I ride my full-sized road bike, but a couple times I rode my folding bike (once each: Dahon MuXL, Tern Verge S11i)
I normally ride a folding bike, which accounts for about 7 miles of the round-trip; the rest is on Chicago's heavy rail (METRA).