I am a MTBer with a road bike I only really commute on the road bike and occasionally go for a little bit of an evening ride.
As regards to ooOOoo's question when I commuted my 16 miles (each way) on a HT with semi slicks (that really needed a good service) it took me around 1.3-1.5 hours now I have my road bike I normally arrive in just under an hour.
Yes I am slow averaging only 16mph but I'm faster going along and up than I used to be and I do think that is down to commuting. Really couldn't be bothered with being a roadie with a MTB as its just not as enjoyable.

