Well, I've done it now and can't recommend it enough. Beijing to Moscow took six days and was great fun. We took the Mongolian route and didn't regret it for one minute.
Few points to note.
Food on the train was terrible, but you could buy from the station or traders. Don't miss the smoked fish from the stations around lake Baikal.
Pay for 1st class. The shower might be cold, but it is a shower!
Once you get past the East of Siberia it gets a little 'samey' in terms of scenery. Most interesting views are Mongolia, China near Beijing, particularly when you go through the wall, and around Baikal.
Try and get a copy of the Trans-Siberian Handbook by Bryn Thomas. It's better than the Lonely Planet guide, but periodically goes out of print. Ours was the only copy on the train and was much in demand.
The most unexpected pleasure was the Hong Kong to Beijing train trip. This takes about 27 hours and was wonderful. You start in the tropical scenery of southern china and when you wake up it's like a whole different country. You also arrive at what may well be the worlds most spectacular train station (from the outside!). Seriously, if anyone flies this route occasionally, consider taking the train, you meet some amazing people.
I'm (Slowly) writing the trip up from my notes and photos. If anyone wants the result then let me know.
[This message has been edited by phb (edited Dec 04, 2003).]