We took a Celebrity cruise to the Panama Canal last year. Because the ship did not actually go into the canal, we took the train ride hoping to see how the locks worked, etc. It was very enjoyable and we felt it was worth the money. BUT, we did not actually experience the Canal. The train did not stop at any of the locks and we were unable to see them from the train.
Since the Brilliance is actually going to Gatun Lake, you should have an opportunity to see the locks from the ship. If that is the case, then I think you would really enjoy the train excursion as a way of seeing Panama, hearing some of the history, visiting Panama City, etc.