What you can get out of One to One is a direct reflection of what you actually put in to it. The person teaching you will generally know a lot about the topics you're going over, but you can't just expect them to do everything for you. Take a notepad with you, or a voice recorder, or whatever method works best for you.. and take notes.
Be proactive about your activities and ask a lot of questions, but try to stay focused on one task at a time. Before you go in, try and figure out what it is you'd like to discuss or learn. You can go over pretty much anything within reason (they're obviously not there to teach you how to use Adobe's CS5 suite).
Is it worth it? If utilized properly, absolutely and without a doubt! Remember that you have to make reservations in advance...
Doug