First of all, I'm so confused by everyone who says that this movie is too long compared to the original Mary Poppins movie....which, hello, is 2 hours and 20 minutes long itself.
Also, obviously for some of the people who have a nostalgic memory of the first movie this one will fall short. Julie Andrews is out of this world, and the original was a magical film. However, it's over 50 years later and this isn't a remake, it's a continuation of the story. Even further, Emily did a *different *version of Mary Poppins than Julie did -- she went based on the PL Travers books and how Mary was portrayed in them, not Julie's more sweet version of Mary in the original. There's no need to compare their performances at all.
Anyway, yes the movie is long and sure there could be places to nit-pick (for me, in some of the supporting cast casting choices) but I took the elementary-aged children I nanny and they adored it and sat through the whole thing very well. They laughed, connected with emotions, and had a great time. They left singing "there's nowhere to go but up" and quoting some of the movie lines as well.
Go see it. It's worthwhile. Have an open mind, don't waste time comparing so heavily, and ENJOY.