I found this plugin very simple but effective; easy to install and easy to use - although it does help to read the instructions as I tried several times to add usernames to the box where you add user ids of those you wish to "log in as" You can also now add the usernames, which for some might be easier. The link is simple to locate beneath the Site admin link wherever you are, and once in a course it is a very handy way to find and log in as a course participant. It is still a drag that once you have finished you are logged out and have to log in again, but that is down to security issues with Moodle 2.
Here is a short screencast: