Eh, it's fine. The writing is okay, if unexceptional, but it gets the job done.
Now, I have to be honest, I thought it was 'made' as an anniversary release, and not simply given away for free to commemorate 25 years of Doctor Who at Big Finish, so throughout the listening experience I was looking for the story to celebrate the eras of the 5th, 6th, 7th and 8th Doctors, only after finishing it did I realize that it was not meant to do that, and as such it doesn't do that.
So, looking at it as simply a story, the setting and worldbuilding of this new race whose entire trajectory of evolution was altered, by the Daleks and the event of the story itself, despite how interesting that may sound, ultimately I think it was built up pretty uninterestingly. Like one would expect from any mediocre Main Range story at the time of release.
The Characters are fine, but almost wholly unexplored, and arc-wise, there's one interesting thing which happens in the entire story.
All-together, it is the usual fare for a 6/10 Main Range release at the time, to a tee, but it just so happens to have 4 Doctors in it, rather than 1. Though the plot is contingent upon there being several Doctors, I still feel like the stakes and implications of the story itself would've been solvable by one Doctor, were they not trying to make a release with 4 Doctor in it. In other words, it doesn't feel big enough to need 4 Doctors.

6/10