My kids -- 7 and 10 -- quite enjoyed this. While some of the cinematic effects are a little dated, they were state-of-the-art of their day -- though the songs and performances all hold up perfectly.
My only comment for parents is that while "Mary Poppins" features plenty of kid-friendly singing and dancing and music and animation, it is a good bit longer than most typical kids' movies (139 minutes, rather than the 100-minute average of today). And, it's underlying themes of class and parenting are quite adult. I don't mean inappropriate but sophisticated. Witty songs on women's suffrage, the influence of the British Empire, and compounding interest will go way over young kids' heads.
So, your preschoolers might get a tad bored. Your tweens and beyond, however, will enjoy it all the more. (My favorite joke: In reference to the Boston Tea Party, "Colonists boarded a British ship and dump crates of tea in the harbor, making it unfit to drink... even for Americans.")