This is where the Harry Potter series suddenly gets some great direction and awesome cinematography. They also never do explain what's going on as well as they do in the book. Hello? Why do these adult people know about the Marauder's map? Oh, you don't need to know that unless you're reading the book. Of course.
Oh god. The best advice I had before watching this was "Think of it as a comedy".
I especially loved how the family looked all shiny and clean at the end - as if they'd all been having tea politely since the aliens arrived and hadn't noticed a thing.