We haven't used it because it is not very convenient for us, but I can provide some details. For owners or exchange there is a fee for use of $5pp (last time I checked, 2 years ago). With a large party, that's more than parking.
At that time, there were 2 or 3 runs to the parks in the morning (outbound only), another around mid-day for pickup or dropoff, and then another 3 late afternoon/early evening (as I recall the last one left before the fireworks too). That was not very convenient for us, and expensive since we already had a car, so we opted not to use it. Thus I can't comment on reliability or speed.
Things can change over time. I would contact the resort directly to find out what the current schedule and cost is.