The past three trips, we've rented a car, and we'll have one this year too. We have stayed at CBR the past 3 times and I have loved having a car there. It is so uncrowded in September, that I'm usually parking in the 3rd or 4th row at the parks and it is GREAT. I also have a double stroller--so it is just wonderful to have a car and not be lugging that thing on the bus.
We do always take the bus to MK since it doesn't really save time to drive there, and it is a lot of on/off the monorail, the tram, and then to your car and with 2 little kids, I'd rather just pop on the bus.
You can get great rates on rental cars. check out mousesavers and click on rental car deals. there is usually a thread on the budget board too for codes.