We rented a double stroller for our last trip (9/09) for DD2 & DS4 (at the time of the trip) from Orlando Strollers. It was great!! We don't have a double stroller, and we wanted DS to be able to sit in the stroller if he got tired. (He has never been one to sit in the stroller - he wants his freedom!)
The stroller was delivered to our resort before we checked in, and it was picked up after we left. We enjoyed having the stroller outside of the parks. We didn't have to wait in line to get a stroller at the parks each morning. We didn't have to worry about them running out. And it was a lot more comfortable for the kids than the hard plastic ones that you can rent from the parks. The seats recline, there was a large canopy to protect the kids from the sun, it was easy for them to climb in & out, and there were times when DD fell asleep in the stroller.
I will say a double is a little bigger than our single travel stroller at home, but not a whole lote. We didn't have any problems taking it on the busses, or wheeling it around the parks, stores, etc.
We are going back this fall, and we are planning on renting another stroller from Orlando Strollers.
Good luck!