[chpatrick]

I'd say that the basic user and session management is easy to implement. The issue is when things get more complicated and your customers want to have single sign on, groups, roles, OpenID and so forth. You can still do it yourself, but at some point you might be spending a lot of developer time on reinventing the wheel instead of working on your actual product. You might also have to build all the associated dashboards for managing all that.