I love the .NET Login controls. They're the easiest things in the world to use--provided you're happy with the arcane and more or less evil Membership tables that are generated by aspnet_regsql routine.
And of course, I'm not.
I often want little things like using the email address as the username. Everyone knows their email address, and in ensures that I collect that information (yes, I'm sneaky).
Usually this means many hours of arcane coding while I implement my own versions of the Membership and Roles providers, when ultimately all I want to be doing is modifying things every so slightly.
This is a concept that makes me ecstatically happy once I found that the source code for the SQL Membership Providers have been released by Microsoft (a fact discovered via Scott Guthrie's blog in this post).
Sometimes I'm so behind the curve.