I am a permitted wildlife rehabilitator and bought 2 of the double and one single to house baby raccoons, possum and squirrels. till they can be released.  I currently have the 2 doubles connected with zip ties to make a large quad.  4 baby raccoons are happily playing.

Pros: easy for 1 person to put together.  Great quality.  Very sturdy.  Easy to clean.
Cons: Having the option to add levels onto top and sides would be wonderful.