What do you think of couples that live together?

Just curious :) I lived with a guy for two years and broke it off, had to move out. Currently dating a guy who wants to live together, and I have apprehensions about living with someone again unless I get married to him. If you agree with it, when do you feel it’s appropriate in a relationship? What do people think about couples co-habiting premaritally, and the increased incidence of it today?