When I was a young scout our district had a Merit Badge Pow-Wow every spring. 3 Saturdays in a row, we would meet for classes. We had to be all pre-registered, so they could have rooms, counselors, and class sizes worked out beforehand. They did three classes each week, and there were 3 session of each class. That left time in the weeks between to do most things that couldn't be done in a classroom setting. The event was free, except for possible supplies needed in some classes. I earned Citizenship in the Community with a member of the City Council, and Leatherwork with the owner and employees of the local leather shop. The district was very carefull to get good councelsors who really knew their subjects. I probably wouldn't have access to that knowledge base if I did the badges on my own or with my troop.
More recently, my current district has started doing a Merit Badge Shindig every winter. 2 Saturdays and longer hours. You can sign up for 6 classes, and each class has 2 sessions. Some of the MBs are taught in 2-hour blocks, so you don't get as many, but you get more time on those harder ones. Last year they added rank advancements, and this year they added lunch (prepared by those working on the rank advancement requirement of cooking). In previous years it was free, but now that they added lunch, they have also added a charge. Some years, the scheduling and counselors was very disorganized, but some classes were very well done and received. This list time, I didn't attend, but they did better advance preparation, so I presume it went better.
Utah National Parks Council (about 40 miles south of here) has been doing their Merit Badge Pow Wow for almost 50 years. They work with Brigham Young University and Utah Valley State College for theirs. Many of the MBs are taught by volunteer professors and college students, using the school facilities. (Chemistry MB in the chemistry lab, Woodworking in the Wood Shop, UVSC Rodeo Club teaches Horsemanship, etc) They get about 5,000 scouts and 500-600 leaders all taking classes on the two campuses. They do 3 classes for 2 Saturdays. (See the article in the September 2004 Scouting Magazine.)