We just got back from Downtown Disney and I'd rate the bus service as good, but not excellent. The buses claim to arrive on a continuous 20-minute cycle, and usually they were quite prompt. Twice we waited closer to 45 minutes for a bus to arrive, once a Magic Kingdom bus, once an MGM/Epcot bus.
The Animal Kingdom bus runs every half hour, rather than 20 minutes, and in off-peak hours it runs every hour on the hour.
The Downtown Disney bus stops at all the hotels, as I understand it, but at the Wyndham Palace we were always the last ones on and the first ones off.
The waterpark buses run VERY infrequently, like a couple times a day, and it's easier to walk to Downtown Disney and take Disney transportation, in my opinion (if it's not too much of a walk from your hotel) at least to the the waterparks.
If I recall correctly, we had bus service to the four parks, waterparks, and maybe Downtown Disney, although we never took it to DD since it was only a short walk. At the end of the day or if you wanted to go back midday, the buses parked in spot 26 at each park for pickup. I'm not sure what time the pickup schedule was, but it seemed like whenever we were ready to go back, they were there.
Wow, that was a lot of information.
Hope it helped.
Brandy