I have a 3-out Netflix subscription I share with my wife, and I setup a Profile for her so she can select her own movies.
I was playing with the Profiles feature (ok, I was sneaking in disc 6 of Lost into her queue) when I received a warning that this movie was already in my queue. I wasn't paying attention and would have had two accounts ship the same movie. This is a great feature and will help prevent duplicate rentals.
I know several people who share a Netflix subscription and use Profiles to give each person their own Netflix account. Each user has their own username and password, and can setup their own queue. Recommendations are different for each Profile.
To setup profiles, visit the "Your Account Page" and select "Add Profile." You can restrict the account by MPAA rating (no "R" movies for the kids), and you can select a different number of movies for each account.
You can also use Profiles to get past the 500 movie limit for each queue. By setting up multiple Profiles, you can have up to 2,500 movies in your queue(s).
Do you use Netflix Profiles? What do you like and dislike about it?