Disney transportation is overall very good except for going from resort to resort. However, a lot of this is in the perception. On our recent trip we used the bus occasionally, but mostly we drove. However, one time we took the ferry from OKW to DTD and then had to take the bus back because the ferry was no longer running. We ended up waiting over 45 minutes to get a bus. This was a little too long in my opiniion. However, the OKW bus stop was the same bus stop as POR. You should have heard these people complaining about how long they had to wait. All of these people showed up at the stop after we did and during the time we were waiting we actually saw three POR busses. So, none of these people waited longer than twenty minutes, but I actually heard some of them complain that they had been there over an hour.
My expectation is that you can have to wait up to about 20 minutes for a bus. This would put the average wait at about 10 minutes. However, many people never remember the times that they walk right up and on the bus.
A car is nice if you like to be in control. However, it is not really necessary except for going from resort to resort. For example, we usually eat at Artist Point once during our trip and one year we were staying at CBR. Well we asked what the best way to get there and we were told to take a bus to DTD and then a bus from there to WL. We left an hour and a half early and were late for our reservation. From then on, we decided that we would not use the busses to go from one resort to another. However, we do still use them to go to some of the parks, especially MK.