We normally rent a car, but on our last trip we decided to save some money.
Never again!!
If you believe that you won't be impatient waiting in uncovered lines at the bus stops (think intense sun or rain), don't travel much to other resorts for meals, don't travel to DTD, or don't mind waiting in tremendously long lines at park closing just to stand on a crowded bus with a lot of grouchy stinky people, or have a stroller or a small child that you don't mind holding while they sleep, then the buses are fine.
Also, we personally enjoy driving to MK vs taking the buses. Yes, it's not as efficient at times especially since Disney generally does a good job of having lots of buses at park opening to the MK. To us, MK is meant to be approached via car not bus. You park in the parking lot and leave behind the "real" world and board either the monorail or ferryboat for your entrance to the Magic Kingdom! We also find it no less efficient at park closing to take the ferry or monorail to our car than to wait for a bus after the fireworks. Plus, everyone is assured a seat. Mid-day trips to/from the MK are about 50/50 on whether or not it's faster. It all depends on how long you have to wait for a bus.
If you park hop, I would suggest driving from park to park. I know some may suggest parking in the park you plan to be at at the end of the day, but we find that less efficient. Yes, there are buses from park to park, but that can be slow at times.
Anytime, you rely on Disney Transportation, allow 30-45 minutes from place to place (this includes waiting times). It may only take 15 to 20, but that's only if you get lucky and walk right up to the bus stop/monorail stop as it's currently boarding. Driving generally takes us 10 to 20 minutes except to/from the MK where you have to allow extra time for MK for the monorail/ferry.
The only times we enjoyed the Disney buses was mid-afternoon where we met some very friendly people & enjoyed virtually empty buses with plenty of seating. However, it did seem that we had to wait 15 to 20 minutes each time we tried to catch a bus in mid-afternoon as they don't seem to come as often.