Let's summarize: The idea is that there are two worlds that are caught by each other's gravity. For some reason, they are in geosynchronous orbit with each other, and the entirety of each world is directly aligned with something on the surface of the other world - so I guess that makes them both flat.
One world is the happy-go-lucky world of privilege while the other world is downtrodden and suck-ass. Inevitably, these two star-crossed lovers find each other. How sweet and extremely unlikely.
This movie sets up rules that are really hard to understand or believe, and then it goes about ignoring those rules as it sees fit. Physics can be manipulated into doing awesome and cool things. There are a host of sci-fi writers would would kill for a couple of A-list actors and a big budget to create a fantasy world that sets up physics and then follows the rules.
The love story is slap-together, and the actors may or may not have chemistry - we wouldn't know, as the shtick with the gravity going both directions seemed to intrude quite a bit. The effects are way too CG and not convincing in the least. As I said, the physics just doesn't work like that.