Folding bike isn't that great. My wife got a citizen bike, rode it a couple times, sold it and bought a Specialized entry level road bike and FAR prefers riding that.

I rode a fixed gear for the first time at the track this last Saturday. It's different for sure, but you get used to it pretty quickly. I'd still say that for commuting it's not ideal. For accelerating or hills you have to stand, which takes more energy than downshifting and spinning, and it's harder to stop on a dime in an emergency situation.

Road bikes are great commuters. I saw the GMC bike all over the place on campus, but I can't speak for the quality. As Tim said, an old 80s or 90s road bike from Craigslist can be a great deal. Often under $200 and in great condition. Even if you don't know much about bikes you can test ride it, and make sure it fits, and everything is lined up properly and looks in good condition - cog and chainring teeth good? Generally clean? Shifts and brakes well? Nothing loose/rattling/squeaking?

I went and test rode a bike once - old Kestrel carbon road bike. Didn't know anything about bikes but wanted a nice one. Loved the bike, but it wasn't shifting well and noticed teeth were broken clean off. Didn't know much about bikes but obviously didn't trust that anything had been taken care of and didn't want to buy a whole new drivetrain. The guy obviously didn't care about the bike and just wanted to make a quick sell to pay his overdue mortgage.
Next bike I looked at seemed to be in good condition. The seller obviously loved bikes, and was selling to switch to mountain biking. Bought it and didn't have to adjust or replace anything for a long time.
First craigslist bike I bought a year prior to that was a 98 Novara Strada (REI brand), name brand quality components, everything was clean and seemed practically new. At only $150 it was a steal. You can get some great deals on Craigslist.