I enjoyed both series (two series of 4 episodes each), but they have been edited for a mass audience and sometimes things get lost in editing. Actually quite a lot does as they filmed *loads* of footage. The guys that do the filming are pretty knowledgable and were great to work with (though my some total on-screen is as a yellow dot during a pan shot), but if I remember correctly I think they said it gets edited by a different team when they get home. Interestingly they did mention that they do different edits for different markets, so the US show will be slightly different from the UK one etc.
Looking at the Duxford one, our show got mixed up with the tank museum's "Tankfest", and I'm pretty sure they re-dubbed the sound on some of the vehicles and mixed up Centurion and Chieftain, but the result is an enjoyable watch and I'm hopeful that they'll do a third series. It's always nice to see more of our hobby on television, and overall it did a good job of being educational at the same time as being entertaining.
As Steveo mentions, a tank restoration does take several years. The Centurion Mk3 is still in the workshops being worked on now with probably another year left to go. That puts a complete restoration out of the scope of most documentaries. It is a shame as it would probably be very interesting.