There have only been one or two times that I have felt that I wished I had a car or taken a cab. And I've been to WDW 30+ times in 10 years.
Yes, there are times the line for a bus is long. Usually at park closing. So, what to do??? Hang out at the park and have a Mickey bar....sit and watch everyone else streaming toward the exits. Then, about 30 mins later, head out. You'll find the bus lines to be a lot shorter.
Park opening??? Sure, there can be a bunch of people in line. But, the buses come frequently the hour prior to park opening until about an hour after the park opens. And yes, you may find yourself standing on the bus a few times. But, it's just not that big a deal.
Even if you have to wait for a bus, it usually isn't longer than 15 or 20 mins. Then you have the actual ride to the park. I usually give myself 45 mins to get to a park in the morning, or back to the resort.
I only use a cab if going from my resort to another resort and time is of the essence.