You can change back at any time but you can only choose one. Think Animorphs but you can choose whatever animal you want. You can't get stuck as that animal and you still have the awareness and thought processes of a human being.
You can choose one real animal and one mythical/fictional animal (this includes animals from movies and books).
Mine:
Real: An owl. Probably a type of horned owl like the Eurasian Eagle Owl. This would open a new world to me - I could fly and have excellent night version. I also wouldn't have to worry about predators really. Not to mention I could experience twisting my neck around nearly 360 degrees.
Fictional: A Hound of Shadow(from Malazan: Book of the Fallen). - Ridiculously powerful to the point where I could take on nearly anything. I'd also have the ability to become invisible and I could call on the other hounds for reinforcements. As a plus, I could travel through warrens to other realms.