I think it really depends on the season and how tolerant you are of waiting.
I've stayed at Downtown Disney resorts and used the transportation with no problems at all. The buses picked up every half hour and were not excessively crowded. They ran from well before park opening until well after park closing.
However, I do know some people have reported really frustrating experiences with the DD transportation system, particularly during high occupancy periods and busy times.
At times when I was on the DD buses, I would overhear people complaining about the bus being late. In some cases I had walked up to the bus stop at the same time as the person who was complaining, and the reality was that we'd been waiting only 28 or 29 minutes (because we had just missed the last bus). However, I think a half-hour seems like a long time to some people, especially when they're tired and hot and their kids are whining.
Personally if I were trying to avoid the expense of a rental car, I would try using the DD buses, and if they turned out to be a problem, I'd just call Yellow Cab. Taking a metered taxi from DD to or from one of the parks should be $10 or less. You can do a lot of one-way taxi rides for the cost of a rental car, unless you get a spectacular rental car deal.
I have lots more advice about using taxis on my site.
Mary
MouseSavers.com