I have done this several times,with a graphics designer, a show producer and even the vice president of WDW imagineering. Each lunch followed the same pattern. There were about 7 total in each lunch(I think they limit it to 12) and we were escorted to a small private dining room(Bamboo room) off the main dining room, were the imagineer was already waiting. On our way through the dining room, we were given some trivia about the history of the real Brown Derby, (oh yeah, by the way its at the Brown Derby at DHS!)and about Disneys recreation. We were each given personalized menus and made our choices from the limited selections(salad,choice of 1 of 3entrees,dessert,drinks,bread)While the orders were taken we went around and introduced ourselves while the imagineer gave a brief career history and their position at Disney. From this point on for the next 1 1/2 to 2 hours(even as we ate) it was a very informal, casual, question and answer discussion on any topic you wanted to bring up. It is always extremely interesting to hear all the backstage trivia and stories about how and why things were done at Disney. The more questions guests had, the more interesting the lunches were(no awkward silence). At its conclussion,while individual checks were being paid(cost was in the $60-70 range), the imagineer signed a special plate for you to take with you and posed for pictures.
My first concern with doing it multiple times was getting the same imagineer twice as you dont know who it is until you show up the day of it. And, the reservation people cant tell you either. But so far this has not been a problem and I have been told that there are 15 imagineers that rotate the lunches depending on their individual work schedules. A big tip is to think of questions and topics(general ones,as you dont know who you are getting) before you arrive while you have plenty of time to think about it. I always sit there trying to think of things that I always wanted to ask a CM, but never think of one until I have left the lunch.
Also, if it is still available, there is a "Dinner with an Imagineer" at Artists point whick I think is in the Wilderness Lodge. It is more upscale and more expensive. There also used to be a "Lunch with an animal caretaker" held at the Animal Kingdom lodge but was unfortunately discontinued several years ago. I never had the opportunity to attend one, but wish they would bring it back.