They usually aren't worthwhile, they increase happiness but if you have a nice communal dining hall that's been smoothed and engraved, enough alcohol, and a half decent cook, then odds are no one will be unhappy anyway. If you are having issues with happyness, mass-bedrooms can be better than an individual room for each dwarf family. Just dig out an area and cram beds into it until it's full, then make each one a bedroom. Dwarves still get the benefit of their own bed, but it takes up a lot less time and space to comfortably house a couple hundred beards.
I like to make rooms for my dwarves, just as a personal preference, but I don't know that I've had a fortress where individual rooms have made a noticeable difference in tantrums/unhappyness problems. It's just very easy to keep the dwarves happy with food and a place to eat, the biggest change Ive noticed is more "ecstatic" dwarves vs just "happy."