I used to commute to work on a Marin B-17...its almost a 40lb downhill MTB with 130mm of travel in the front. It was hard, but I have never been as strong since stopping. Upgraded to a GF Marlin with 60mm travel and lockout, was awesome. Wish I switched to slicks, but the suspension was a nice option. Currently rebuilding a mid-90s rigid Univega for commuting...I figure it will fare better than my Motobecane. I'd say its a personal choice, depending on your fitness and desire to work.

IMO, go for it. Your body will thank you, eventually.

EDIT: my commute was only 4.5mi each way back then and I was 16/17, now its 17mi each way and I'm 22.