Saw this double feature last night. It was great. Mary Poppins on the big screen was a real treat -- I had never seen it in a theater. And Saving Mr. Banks is a brilliant film. It has great acting (If Emma Thompson doesn't get a best actress nomination, I'll be shocked; Colin Farrell and Tom Hanks could see Supporting Actor nominations as well), a wonderful script, heart and humor, and enough Disney-related easter eggs to make us Disney nuts giddy.
Go see Saving Mr. Banks whenever you can. But if you want an added boost of enjoyment, watch Mary Poppins in few days before you see Banks. There are many little nods to Poppins in Banks that you'll miss without a very recent viewing of Poppins.