I chose no, because I don't really view a couple as married unless they've made the conscious decision to actually get married. My husband and I have been together as a couple for a little over 15 years, but we've only been married for 5.
Now, as far as common law is concerned, there are legal rights you may have depending on what state you live in. I did some research a while back (before we were married) and it seemed that people mostly invoked common law marriage in instances where one of the parties passed away and the other was looking for survivors rights. In most states, when you die all of your assets automatically pass to your spouse unless willed otherwise.
In the state we lived in at the time, there were other criteria beyond having lived together for a long time. If I recall correctly, to be considered married under common law you also had to prove that you generally lived as a married couple (filed joint tax returns, had joint bank accounts, used the same last name, etc.). It was complicated and convoluted, and really amounted to trying to prove something after the fact.