I've rented a few times before, neither time entirely satisfactory, but for different reasons.
One shop insisted on running some heavy tires that had about as much traction as a dog on a freshly waxed floor. They said it was for puncture resistance, but I had trouble modulating the brakes going downhill - far too easy to lockup on dewy pavement.
The other shop had no such requirement, and the bike performed fine (Specialized Roubaix), I just ended up disliking the bike. Couldn't get used to pogo-ing on the Future Shock, their Di2 'chirped' when shifting, and the saddle nose somehow tore a hole in my shorts. I liked the disc brakes though.
Whatever you do, I would definitely suggest bringing your own saddle and pedals at least - in fact, some bike shops only do pedal-less rentals.