I would definitely give it a try, since it's free. AS long as I have a seat on the bus, and not have to stand, how bad can it be?
I used Mears twice, a town car once, and a limo twice. Of course the limo was the best, but we didn't "need" to use one. It was just to make the vacation more exciting.
The first time with Mears, we were on a coach bus, and it was actually a pleasant experience. The second time, it was a van, and I wouldn't want to do that again. It was uncomfortable to be sitting so close to people.
So if the free Disney transport is a coach bus, I'd try it. I want to get to my resort as soon as possible, but it wouldn't be unbearable to make stops at other resorts and see them for a few minutes, especially the ones I'd never seen before.