Hiya,
Yep, if you want a bike, get a bike. You won't ride it every day to work, maybe 2 or 3 days a week, it won't pay for itself, it will require far more attention than a used Civic or Corolla, it won't get better mileage unless you get an old Honda Trail 90 or a CB-125 at which point, the trucks will smush you flat.
I know I sound like I'm anti-motorcycle, I'm not. I currently have 8 of 'em, a Buell, Shovelhead, Sporty, couple Can-Am's, Yamaha and a couple Maico's. I've been riding and racing since 1971, I've raced every type and class at one point or another, MX, enduro, scrambles, ice, trials, hillclimb, speedway, flat track, supermoto, road and drag racing. Between me and 2 friends, we have had more than 127 bikes over the years. (That's the count of the ones we remembered)
I rode the Buell to work once last summer, I couldn't wait to get back home. Commuting is vastly different from going out for a ride, the speeds are higher, the drivers are far more aggressive, everything takes place a lot faster and the room for error is a lot less. I was planning to take the Shovelhead but never got around to it, the AC in the car sounded like a better idea.
Like I said, get a bike if you want a bike but don't get it for commuting as you will do it once or twice and park it. Motorcycles are meant to be raced or ridden for fun, not to go head to head with cages on the highways during rush hour. If you do decide to get a bike, I would suggest a 1991 and later Sportster 1200. That is the first year of the 5spd sporty and the 1200 has enough power to hold it's own accelerating, it brakes and handles good enough to keep you out of trouble and if you loose interest in it or want to trade up to a big Harley, you can sell it for what you paid for it or maybe even more. I see them all the time on CL for about the $2500-3000 range.
My 2 cents,
Tom