I'm a fan of cheap and simple. I found well respected steel mountain bikes on craigslist, like Specialized Rockhopper or Hardrock, and sweetly, my Miyata Ridgerunner. These cost around $50 to $70 each, and make great commuters. I added fenders, a rear rack, used panniers off eBay, a bell, and a super blinky on the back. Total cost could be about $100, and you can keep the rack, fenders and saddle bags should you ever upgrade. I found commuting to be super for my health, my mood and my wallet. Don't become a bike consumer. Stay cheap, use regular clothing and enjoy yourself. I do about 5,000 miles a year, and my only real cost is about $100 a year for tires.