We've always had luck and good experiences with the resort wheelchairs and that saved us a lot of money while traveling with family. That said, we haven't used this service since 2006.

However, we also had luck with renting from a couple of outside companies who delivered right to our non-Disney property resort. We found companies outside of Disney did have the nicest models of non-motorized wheelchairs (never rented the motorized).

Depending on your plans, you could always see what sort of shape the resort wheelchairs are in first (say, if you're not going to the parks the first day). Then, if they're not satisfactory, you could setup an alternative for the next morning. Also, with borrowing from the resort - the staff was always nice about letting us keep them overnight in the room without re-checking it out each day, but I am not sure if they still do this.