We did cars our first three trips, and now we do buses. This is why it works for our family: your mileage may vary.
We find the trip to get to the car to be more stressful and more exhausting than the trip to the buses. We find the buses to be a little easier and more accessible (why on earth do they make you walk the long way around those barriers at MK!?!). The trip to the car at MK is way too long and has too many transitions, and that is our favorite park. We stayed at WL once, and enjoyed the boat option. When we used the buses, we found that our son had the same tantrums he did when we left to go to the car, but they'd be over before we got to the car in most places.
We stayed at POR, and didn't find ourselves having to wait very often, though it did happen. We also had some pretty nice social interactions on the bus. My son even shared his toys with kids from another family, and then had a little girl accidentally break his brand new monorail toy on the bus one day. I had visions of the bus being stopped, police cars, etc (my son can put on an impressive meltdown), but some sort of Disney magic intervened and he handled it beautifully and was actually able to hear me when I told him we'd replace it. (Her dad started shoving money in my pocket and I kept trying to give it back to him because it was about twice what the toy had cost and I told him I wasn't going to make money on this!!!)
The thing we liked the most about the buses is that it gave us some freedom to split up as a family. My AS son has a NT twin sister, and we were doing a lot of balancing people's conflicting needs when we stayed offsite with a car. Last time we stayed onsite, and we were able to leave when he wanted to leave, and his sister could stay later (this presents issues when you don't plan for an afternoon without the GAC, but we've corrected for that after a few days of doing it wrong!). I suppose if you were onsite and had a car, you'd be okay to do that too. We also gave ourselves permission to spend up to $150-200 for taxi rides, and didn't come near that, though we did use a taxi after IllumiNations.
The other thing for us is the car rental process. The plane ride takes a lot out of our family, and we've found the rental car process frustrating. We have paid a premium to rent in-airport; other times we've taken shuttles off airport, and both involve a huge amount of waiting and hassle and running around with all our bags and kids in tow. Sometimes you hit a great sale and then find out that everyone else on the east coast has done the same thing and you wait an hour just to get your keys (true story).
Then, on the trips where we did split stays, we found ourselves leaving the car in the parking lot for days at a time (and at WL, the parking was not at all convenient). The combo of ME+buses+taxis seemed cheaper and less of a hassle. And our experience with ME was pretty positive. I know others have not always had that experience, and we've only used it once.
Of course, staying at the Contemporary is the ideal solution. Except, as I tell my kids, I'm just a thousandaire, not a millionaire.
Hope this helps!
Cate