So far everyone has said that they like to drive. That surprises me because there are lots of WDW transportation lovers out there. Count me in the ranks. We drive to WDW, but once we park at the resort, that's it. We use their transportation the rest of the time.
I like being able to sit and wait for a bus in a shady waiting area with fans instead of waiting for a tram and walking across a busy parking lot. We don't go in the summer, but the buses are always a nice temperature when we get in them. I'm sure they're air conditioned. When you get in your car, it's going to have been sitting out in the sun, and it will be HOT. The buses are nice and comfortable. You also don't have to worry about how to get there. There are lots of signs so that it's not too hard to find your way around if you drive, but riding is so much easier. DH and I use the time in the bus to plan our attack for the park. It may take a bit longer on the Disney buses if yours isn't there when you get to the bus stop, but IMHO, it's definitely worth it.
My advice would be to try Disney transportation when you first get there. If you don't like it, use your car after that. But give it a chance. If you try it on your last day and love it, you'll want to kick yourself for using your car the whole time.