They can be super helpful...
But it depends on both you and your instructor!
In regards to you, if you want to get the most out of your privates, make sure that you come with a gameplan. Pick a few things that you want to work on and drill/work the heck out of them. Go over a series for an arm bar attack, or a series for a side mount escape and work all the possibilities...
Try to say on one topic and learn all you can about it...i find that it helps you retain the info better than jumping from topic to topic with no real flow, but hey, that is just me
It can also work really well if you roll with your instructor, and before rolling ask him/her to focus on your game, areas that you feel esp weak, and once your instructor has identified these issues you can work on them together!
hope that helps