I returned two weeks ago from a trip to China which included a short stay in Tibet (it wasn't the Viking tour nor did I go on a Yangtze cruise - I just happened to check the Viking message board and saw your post). I found Tibet absolutely fascinating and certainly worth the time and effort to visit. I'm just not sure it's worth what Viking charges which is substantially more than what it would cost to make your own arrangements. Tibet is relatively inexpensive to visit once you're in China. We flew from Chengdu to Lhasa, spent 3 nights in Lhasa, and then flew out from Lhasa to Chongqing . We visited the Potala and Norbulingka palaces, Jokhang temple, Drepung monastery, the Tibet museum and the Barkhor. I posted some of my pictures on ofoto if you'd like to see them:
Let me know if you have any questions and I'd be happy to try and answer them.