Before trying this plugin, I tried another one in the same category.
With the other one, one had to log on with the username of the target user, using the admin password. Logging on as many users became a big thing.
This plugin, however, does not require any password to be entered. Once logged into the backend, you just click the target user, click the button to log on as that user, and bang, you're in as that user. Excellent!
If you're more into typing than clicking, you might prefer one of the other extensions, though.