Bicycles have evolved very slowly over the past 130 years, when the chain-driven "Safety Bike" was introduced. In recent times, the changes have been subtle: weight savings, easier shifting, and other refinements, but the basic design is intact, 2 wheels, a metal (O.K., carbon) frame, seat, handlebars and a chain drive. And as long as the wheels use ball bearings, the fundamental efficiency hasn't changed much either. So what I am implying, is that any bike you can get your hands on is going to work for you in the ways that count: good exercise, and a good way to travel.
Bikes are designed to fit specific jobs or conditions. Road bikes are the best choice for tar roads, mountain bikes are designed for mountains. Pick a bike that is designed for what you are going to do with it, not for how cool you think it looks, or you may regret it. A cruiser bike is so-o sweet looking, but you are going to be walking it up steep hills. A mountain bike is like a rugged SUV, but the price you will be paying is major road resistance from the knobby tires and low speeds from the mountain gearing.
That said, I use a road bike in the Spring, Summer and Fall, and a mountain bike all Winter. The extra sand, gravel, and salt on winter roads can cause thin-tired road bikes to loose traction at exactly the wrong times. Fatter tires allow you to safely cycle on the layer of dirt and move onto the shoulder when you need to. I switch to studded tires when the roads begin to get icy.
A mountain bike is not ideal for a 12-mile commute, but its what I have. Someday I will buy a cyclocross or other style that has the wheel clearance needed to use the studded tires, because I prefer dropped handlebars and higher gears. I like dropped bars for all the reasons they were designed; changing hand positions for comfort, efficiency, and aerodynamics. Mountain bikes come with straight bars and too-low gearing, but somehow, they are still fun to ride every day, and the time difference isn't even that huge. My records show my winter commute takes anywhere from 5 to 25 minutes longer, depending on the weather, which can be severe sometimes. I check the weather report before going to bed, and plan my time and clothing accordingly.
More tips: If you want a practical commuting bike, you should be looking for threaded mounting eyelets on the front forks and rear stays for adding luggage racks and fenders. You should be looking for a range of gears appropriate to the terrain you plan to bike in. There are more commuting-style bikes being introduced all the time, and they all look good to me. Well except for the hokey coffee thermos attached in place of a water bottle. Drink your coffee before you leave or after you arrive. You aren't going to be stopping for coffee breaks on the way to work. Really.
Where should you buy your bike? Online, at the local bike shop, or what?
Online can be a real good deal, but there are caveats. If you know what you are getting, and who you are buying from, and have good mechanical skilz, you might buy an awesome bike for the money. But let me tell you my story. I got a fantastic deal from a reputable California bike shop's online store. The bike arrived nearly completely assembled, and all was well. However, about 6 months later, the deraileur hanger (the little, removable piece of aluminum that the rear deraileur mounts onto) just twisted a little under load as I pumped up a steep hill. Since I was in first gear, the deraileur was already a fraction of an inch from my rear spokes, and this little twist sent it in to my wheel, where it was promptly snagged, twisted and torn off my bike, breaking off the derailleur hanger. I checked my warrantee when I got home, and it stated that I had to send in the bike so they could see if it really was covered. I wasn't about to pack up my bike, and ship it out to California (from Vermont) for up to a month so I took it to my local Bike shop that sells the same brand. They told me that the hanger that broke was a known weakness with Jamis bikes, and they recommend replacing them at purchase. As it was, I paid for a new hanger ($35!), a new deraileur, and a new chain. They installed everything and trued my wheel. $180. uck.
Bike swaps. If the timing is right, a bike swap hosted by a good bike shop can have some incredible deals. These often feature bikes purchased from and maintained by the bike shop, and you can get some amazing deals if the previous owner has traded up and wants to get rid of his last year's bike. I've seen some untouchable bikes sold at very touchable prices. The tip? Get there before they open, the best bikes go first.
More later.
