They are a nice complement to bodyweight training, allowing you to add some resistance to bodyweight basics like the push-up or the squat (or to add assistance in the case of the pull-up / chin-up )
They are not nearly as good as free weights for certain things though. The strength curve is very different to that of a weight and it's not as easy to progressively overload (ie, add weight to) the exercises over time. You can use thicker bands, change your grip to play with the band tension or perform harder exercises (such as one-arm variations of whatever you're doing), but nothing's nearly as easy as adding heavier plates to a barbell or picking up a heavier dumbbell.
They can be a great complement to other equipment, or if you're fully sold on calisthenics / bodyweight training, they might be everything you'll ever need. If however you're into traditional strength training or bodybuilding, they will fall short fairly quickly (personally, I can't wait to get back under the bar).