I detest the Disney bus system; we always get a car, for several reasons.
First, there's Magical Express. I LIKE DME getting to the resort (who can complain about not having to deal with your suitcases?) but going back to the airport is a pain. They pick you up 3 hours before your flight, and it's annoying to have to do RAC for your suitcases. We find it much easier to have our own vehicle for getting back to the airport.
Then there's the hassle at park closing. I hate nothing more than standing in line, waiting for a bus, for 45 minutes after I have been walking in a theme park for eight hours. It's just a killer for my feet and back, and absolutely miserable in the heat of August or on a windy, chilly January evening. You'd think that once the bus comes, it'd be easy, but it's NOT. More likely than not, I am going to be jammed into a bus, like a sardine in a can, with a whole bunch of other hot, sweaty, tired, cranky people. It's hard to stand on a moving bus, especially if your arms are filled with sleeping children or the little ones are awake, standing, but not happy. Then, you get to your resort and get to drag through the lobby, past the pools, and to the elevator and down the hall to your room. I'd much rather hop a parking lot tram, walk down the aisle to my car, and sit down, listen to music, and drive to the hotel- where I can park pretty much outside my room. Even at MK, when I have to take the monorail (or boat) to the TTC and the tram to the parking lot, it's STILL better than standing in one spot, not moving, and then being packed into a bus. Actually, after three trips where we used JUST Disney transportation, DD started asking to ride the monorail and the parking lot tram "for fun!"
I like having my car parked pretty much outside my door. It's quicker and easier on the feet to dash down to a food court to get coffee in the morning. We leave our mugs in the car and stop by the food court on our way back to our room at night, to get a last drink or a bedtime hot chocolate. I'd NEVER bother to walk all the way down to the food court, at bedtime, after a day on my feet, to do this. I like being able to go to Publix and pick up b'fast items, water bottles, snacks, and wine for the room, and to go to the Disney outlet stores for buying souvenirs.
THEN there's the issue of ADRs at resort hotels. If you are in a theme park prior to dinner, that's fine, but if you are at your hotel, trying to get to another, it can be a nightmare. It took us over 2 hours to go from POFQ to 'Ohana, and we were late for our anniversary dinner. It was almost 90 minutes first thing in the morning, to take a bus from ASMu to Epcot to AKL for our ADR at Boma. These are each 10 minute drives, unless, of course, you are waiting for the Disney bus system to get you around.
I know that many people think it's too much hassle to drive while on vacation, but we don't, and we find the signage at WDW to be pretty good. WHat I find to be a hassle is spending too much time standing in line, waiting for a bus, riding a bus, changing buses to connect to other buses, etc. For as long as we can afford to do so, we'll rent a car while at Disney- although I will miss DME from the airport to the hotel, and the luggage handling. I LOVE checking my luggage into my home airport and POOF

it disappears and reappears like magic in my hotel room!