PrivateContent is probably the easiest way to create an users database into your wordpress website. Seamlessly integrated, is based on user categories: then no more basic roles, you can virtually have unlimited levels!
Each user can be assigned to one or more categories giving you the maximum flexibility possible, this means complete freedom in creating simple or complex patterns to properly store them and eventually restrict your site contents.
The plugin has got also some extra features to help you manage users with ease, such as category filter, data sorting and an internal advanced search. Gives also the ability of deactivate users, to keep them in the database but deactivating accesses to reserved areas.
Finally you can export user lists in Excel (.xls) or CSV format, import them using a CSV file or also import existing WordPress users!
This is really useful having to deal with large existing databases.