My wife and I rode in the cab with the engineer on a beautiful Colorado afternoon in early July 2011. It was an experience I would describe as "epic."
Looking at photos of the train online, I was concerned that the windows in the cab may limit the view. Wrong! The views were spectacular from the cab (though the engineer did confirm that the best views are probably in the open air car). I am puzzled at the people who did not find the scenary impressive. You are in a narrow canyon with a river along its side pretty much the entire length of the trip. You have an unmatched view of the Royal Gorge Scenic Bridge. You may see bighorn sheep and other wildlife. In short, the views were awesome.
Regardless of the views, being in the cab was trully an amazing experience. Listening to the engineer communicate with the conductor, watching how he operates the train, learning about the train signals-- all very interesting. The engineer even let me sound the horn once-- probably the single best moment of my vacation, what fun!
If you are in the tourist trap that is Canon City, this is a nice way to get away from the crowds and relax for a few hours. If in the cab, it is just you and the engineer, so no screaming kids, scrambling to get a seat with a good view, waiting in lines to board the train, etc. In sum, being in the cab was well worth the extra cost and made a vacation "activity" an "adventure" that we will never forget. Thanks for the memories!