It depends on your use case...
A controller is required if you have more than 24 APs or want to mix different models of APs in the same deployment (eg: 7502E and 7522E). In this case you would need VX9000E, NX5500E, etc.
If you want full Enterprise features, you cannot use any Express products in the deployment. The same rule applies as above, if you have more than 24 APs of the same kind or want to mix different models (eg: AP7532 + AP7502). In this case you would need VX9000, NX5500, etc.
