Using the cash lanes is easy, but the fees for using Sunpass through rental car agencies vary, and some are better than others. I usually rent from Alamo, and they are pretty fair - they only charge you on the days you use the system, which for me is only to and from the airport. So yes, it's nearly a $10 surcharge for the convenience (plus the tolls), but I like not having to slow down and not having to worry about having change ... so I usually opt for it. Other Disney related sites have some detailed breakdowns of each of the rental car companies charges for toll roads.
And as mentioned above, it if you go semi-frequently, it is really easy to buy your own transponder and just use that.