Decent watch, probably won't watch again, but can recommend, particularly for younger audiences.
It's definitely inspired by Nancy Drew (they drop the name somewhere as a joke), but isn't one itself.
It's a good coming of age / identity story about investigating a mystery: it's somewhat funny, but the premise is that it's stuck up little rich kids (mostly) that have to get a taste of the real world.
Sadly, kids and adults, Lindsey Lohan pretty much carries the movie. She does a good job but other actors really had a chance to stand out in this and just didn't.