I attended ATOP back in July of 2000 when it was still being operated at the United Airlines Training Center (or Training Kingdom as they called it) in Denver. At the time I was a private pilot with a little instrument experience. I just wanted to do it to gain insight to what the training environment was like at a major airline. I was really impressed.
The two-day course is taught by Wayne Phillips, who is a great guy and excellent instructor (you might recognize him if you read Flight Training magazine because he writes aviation career and collegiate aviation articles). You begin by doing a tour of the facilities and checking out the simulators. It gave me my first glimpse at a level D simulator, the best being the 777 with the avionics on. After that you go back to the classroom and begin an intro on the systems of the 737-200 (at the time). It is really interesting to learn the basics and it is greatly beneficial for future systems classes. After lunch, you run through some mock scenarios and start-up flows in the procedure trainers to get a feel for things. After a little more ground school you are done for the day and he assigns "homework" and lets you know what to expect the following day.
On day two, we completed our intro ground school and had an airline career forum where Wayne gives you insights to the industry, career paths, what to expect, interview tips, etc. This was a lot of fun and is a real motivation boost for the apsiring airline pilot. After lunch, you get to go "fly" in the sim. You have a partner during your sim session and you switch off as Captain and F/O, 30 mins of each. We went through the normal flows and shot an ILS into Denver, the whole time Wayne is sending system failures and weather anomolies your way to see how you react. It was a lot of fun having an emergency thrown at us and having to resolve with the proper methods. It's also a great intro to using CRM techniques on the flight deck. After your ride, you remain in the sim and sit behind the instructor station to observe the other pilots, which allows you to see the crew operations from another perspective. Overall, I absolutely had a blast. You get great insight to the training environment at the airlines, get to see what the level D simulators are like, meet some great people (networking), and best of all it allows you to boost your motivation and renew the pursuit of your dream! If you have the money and the time I would definately recommend it.