We've rented with Dollar many times -- both in Orlando and at other airports. We've never had anything other than good experiences. The one caveat is that last time we were at Disney we got a flat tire on the car the second night. We had to bring the car back to the airport or wait for them to bring a new car out the next day. Of course we weren't going to miss anytime with Mickey, so we took the ride to the airport to change the car. Meanwhile, I believe that National is onsite at WDW. So if anything like that happens, you can do the trade right there.
Our checkins at Dollar have always been very quick. I notice while we're checking-in, the National desk right next store always has a very long line since it is WDW's official car rental agency and a large number of Orlando airline pasengers rent through them. I have also always found the rates to be less at Dollar. We are renting an intermediate car in Sept for 4 days at $22.99/day. I checked the site everyday and this rate popped up one day and was gone the next. If you see a car rental deal jump on it right away -- ask questions later. Unlike airline reservations, you are not paying for your car rental upfront -- so, they are extremely easy to cancel and change.
Returning the car at Dollar has never taken more than a few minutes.