Definitely get an ADR as they only serve from 2-5 each afternoon, iirc. Absolutely worth it! We usually get the Buckingham Palace, which is a bottomless pot of your choice of tea, traditional selection of miniature tea sandwiches, scones w/ jam and clotted cream (YUM), and your choice of either strawberries and fresh whipped cream (YUM!) or two small pastries you pick off their trays.
It's a lovely, relaxing, quiet atmosphere. You can probably rush through in about 45 minutes if you were in a hurry, but I'd encourage you to take your time and spend the hour, hour and a half there.
We often do this as the last thing at WDW before heading to the airport. It's a great way to wind down and transition to the hectic trip home.