The narration is very good on the few I have listened to, and it's only very, very rare that I get a bit lost about what's happening.
My only reservation with The Dalek Masterplan is that chunks of the Christmas episode are structured as a pastiche of a silent movie: lots of Keystone Kops action with captions on screen. That does not translate well to audio, even with Peter Purves doing a sterling job of narration. Sequences such as Katarina's death are gobsmacking.
Stories like The Faceless Ones are given a totally new lease of life on CD. I would never have bothered to watch the two surviving episodes (eps 1 and 3 if memory serves) more than once, as they are so isolated from the story that almost have no meaning. However, being able to fill in the gaps with the soundtrack/narration combo is a real Godsend.
There are whole chunks of DW we will never get to see ... and it's all accessible on CD.
I've listened to a few so far which have surviving episodes (Masterplan, Faceless Ones, Moonbase, Toymaker), and have been able to piece together the story from DVD and CD. Wonderful stuff. I thought Toymaker was crap, frankly, but most of the rest have been bliss.
Marco Polo, The Highlanders, The Massacre ... we'll never see any of those stories, but they are all worth a listen. (I thought The Highlanders was summat-and-nowt, but The Massacre is really powerful).
Treat yourself!