I have used the Mears/DU offer many times. It has worked out fine. We even used it once to get over to Royal Pacific Resort to spend the night..used only backpacks since luggage is not allowed on the bus. We were able to get from our WDW resort to US/IoA early enough to walk to the RPR, get checked in, drop our stuff off at Bell Services, and head to the front gates....and got there about 15 mins prior to park opening at 9! I believe we got the 7:30 bus.
Now, that was on a weekday, in probably December. Sept is going to be fairly uncrowded. Try to get a pickup time of about 7:30. That has always had us arriving in time for park opening.
Does it take longer to use Mears vs a towncar? Yes. The towncar is going to take about 25 mins to get to US/IoA. But it will cost you about $50 for one way, and around $90 r/t. The Mears shuttle will cost you $19 r/t, and will take up to an hour to get there in the morning, but probably over an hour for leaving. I've had very good luck in that it has never taken me more than an hour to get back to WDW after a day at US/IoA. Then again, I tend to leave that area mid to late afternoon, vs park closing.
My one suggestion is this...do NOT make an ADR for the night you return from US/IoA unless you are planning on leaving prior to 4ish. You could find yourself on a bus that has to make many stops, in some out of the way areas, which could have you late for an ADR at WDW.