I use a MTB for commuting, an old Trek hardtail/fixed fork model. I wouldn't want suspension. I put slicks on it, but otherwise it is pretty much stock. In comparing commute times on my 10 mile commute between this bike vs. my 16 lb. carbon road bike, it may make a 5 minute difference, mostly because I can ride uphill faster on the road bike. Otherwise, the traffic and road conditions determine my top speed. Really, I can't ride at 25mph on Manhattan streets on my roadie, in fact I really have to take it easy to feel safe.

I'd say go for it. (btw, my mtb cost me $150 used, I wouldn't spend too much.)