Oh, one other thing I can verify is that you must scan in to a park to be able to book a DAS return for that park. If you are hopping, you must scan in to that park before making a return time there. And, to clarity, the DAS tile does not show up for your first park until you scan in.
We also experienced after having used a day-of DAS return, that sometimes they don't "clear" and you cannot make a new one until you can get either a ride CM or a blue umbrella CM to clear it for you. The blue umbrella folks can also make a new one for you and seem to have a bit of leeway with what time they can do it... We found that a Everest DAS did not clear, but when the CM tried to make a new one for another ride for us after clearing it, it was after when we planned to leave, and she was able to bump it up to a time that would work for us...
AND, we had trouble doing the online DAS booking during the early entry, but ride CMs and blue umbrella folks can do it for you.
Again, we had a great trip that we would not have been able to have because of the DAS program. It really worked well for us after we figured it out.
I'll be happy to try to answer any other questions I can...
EDIT: One other thing I just remembered.. they do use DAS returns for the extended hours for Epcot and MK, but NOT for the AH party events.