I liked the characters, and I felt that the story was well developed, along with the storyboard, camera views, etc. Voice acting was great. My only suggestion is to work on animating the characters consistently well; many scenes were great, but there were a few that didn't have enough facial expression to go with the emotions of the characters, or sufficient body language. Additionally, avoid simply rotating characters without changing their arm and leg position.
Other things to work on: Rex's eyes stayed vertically still, when his upper face was clearly moving, in certain scenes when he talked.
I also felt that some of the backgrounds could have been more detailed, or for the ones that looked fine in detail, seemed a bit sloppily drawn.
Anyways, it was overall a good movie, and I really like the style/appearance of the characters- it has a nice comical feel.