Everything you wanted to know and weren't afraid to ask.
Maybe you would, maybe you wouldn't, but you probably want to meet people with whom you have something in common. Similarity allows you to decide for yourself what that something is.
Look carefully at the match tuning page. This is where you decide what questions are important to you. For example, you could say that hobbies are less important than political views and musical taste is irrelevant.
Feel free to try different configurations and see what works for you.
The first incarnation of Similarity had three different sets of radio buttons that you had to click on for each question: your response, the response you prefer in other people, and the importance of the question.
It sucked.
The current approach (only describe yourself, with the ability to adjust the importance of categories) is a compromise of usability and functionality. At least it's no longer painful to watch new users fumble through the questionnaire. Sorry.
The first version of Similarity went live mid-2001. It was not a great success, so I never put it back up after I moved to San Francisco in 2002. It has been a backburner project for several years, but I recently took about four months of free time for a ground-up rewrite. What you see now went live June 2005.