A full quarter of the sites I've customized always ask for such a simple feature: the ability to quickly return to the admin user account after logging in as a regular user

It's really quite simple to implement:

Modify admin login-as-user logic to store the current admin UserId value to a session variable, authenticate as selected user and redirect to store home page

Modify shopper-side header to render additional 'Return to Admin' icon if session variable exists

Clicking 'Return to Admin' pulls in the session variable and validates the UserId value, calls Forms authentication to re-authenticate the admin user, clears the session variable and redirects back to admin side.

Such a simple feature that gives so much benefit to store admins.