+1 for converting the mtn.

I found an old Trek 830 on craigslist which I converted into a commuter and looooove it It's not perfect but it's pretty darn close I still have bike envy of the other commuters posted But perhaps I will upgrade later when I can better afford it

I invested in new tires, panniers, tubes, etc. Fortunately for me, it came with front and back lights, a frame pump, a rear rack and a lock

Then when it wears out (if it does) then you can get a shiny new commuter... OR you could let the mtn bike be your commuter and get a beauuuutiful new bike and keep it inside away from the harsh elements