I rode the last ~4000km on the Silk Route 2007 ride. My trip report is: http://www.crazyguyonabike.com/doc/mevchina2007

My suggestion is to find and read some of the other trip journals out there. You'll find different expectations about the trip and different perspectives of the trip. There will also be differences between Africa, Silk Route and Orient Express. It is even amusing to read different accounts of the same trip.

Personally, I was fairly impressed with TDA as an organization in being able to adapt to changes and run an overall expedition type trip. At the same time, having the right expectations is also extremely important of what type of trip you might have - since there are some folks who wouldn't necessarily mesh with a TDA trip and also some folks in Silk Route 2007 that didn't necessarily anticipate the type of trip that they were on (e.g. some folks were surprised to spend ~85% of the time cycling through desert regions in the Silk Route though arguably they could have figured this out before departure).

For me personally, I'll choose to ride across South America on my own rather than go with TDA... but I've also put down a deposit for a future TDA trip through Africa. That says even for one person, I think enough of TDA to do another trip with them through Africa, but I also realize that a self-supported trip will be better for me in South America.