I'll chime in here since we just got back yesterday and flew in - did not rent car. We never had more than a 15 min wait for buses - and that may even be over estimating time. Early in the morning we did walk to main bus stop at Riverside entrance because it stopped there first, or so I thought. PO only shares bus with French Quarter, but not all the time. On some occasions, it bypassed French Quarter and went straight to Riverside. I think if varied based on demand. We also were on the bus when returned to POR an it bypassed the main stop and started with our stop - the West Depot. So I think there was quite a bit of communication going on between drivers that adjusted stops based on bus loads and frequency of buses.
Waits for return buses were not that long. I think on one or two afternoons we may have had a 20 minute wait.
Several times we had to stand, but everyone was minding their p's & q's and willing to give up seats for moms with babies, small children, or elderly. Even teenagers were requested by their parents to give up seats for adults!! (Way to go Parents!) With the exception of Animal Kingdom, buses went straight to parks. Animal Kingdom shared with Blizzard Beach.
We've never driven to the parks (even though we have driven to WDW on 3 occasions) and always used their buses - so I can't really give a comparison. But with the short wait times, I imagine that must be less time spent from resort to parks if one were to drive to park parking lot and catching a tram to entrance. I may be wrong.
I just wanted to point out that we have very good luck with buses and we never encountered over hearing anyone else complaining about waits. Also, judging by the small crownds at the bus stops at both resorts and parks, I can only surmise that buses were running quite frequently.
BTW - I noticed bus stops at the parks that go to the other parks. I always thought that if you wanted to park hop, you had to take a bus to a resort then to the next park you wanted to visited, but apparently you can now catch a bus that goes from one park to another. I don't know this for sure as we didn't park hop - but I did notice these labeled stops at Animal Kingdom, MGM and MK, I believe. Maybe someone can confirm this.