I've rented and also traveled with my own bike. Of course it's best having your own bike but if you travel to a place with a decent sized cycling community you can find good bikes to rent. If you do rent, make sure you know exactly what bike you are renting and which size. In preparing for one trip I confirmed the rental bike several months in advance. Still, as the date came closer I called again and they tried to put me on a different bike. I insisted that I get the bike I'd reserved and in the correct size. I ultimately got the bike I originally reserved, a Trek c-fiber frame. I would have liked my bike better but the Trek was actually very good.
Bring your pedals and know how you want the bike to fit. A good shop will take care of you. Oh, I've found that some shops have a daily rate, and if you rent for 4 consecutive days they give you the rest of the week for free. Do your homework and ask the right questions and renting is a viable option.