It's a really nice upgrade! Aside from the faster processor, the quad speakers, pencil support, 12mp camera and improved screen do a lot to separate it from the last gen models.
I preordered one as well and have been using the mbair sine the since the og mini which didn't get much use. Used powermax,com to trade in. They give best value but you have to spend the credit in their store. Further I don't have to send in my mb until after I get my new iPad Pro. I am one of the first on their list and they are an authorized Apple retailer.
Update: I'm glad there is the 7 day window between preordering and delivery. It gave me time to do need vs want and I realized my Air 2 is perfectly fine. I've cancelled my order. Perhaps another time.