I am very found of Colin Baker's Doctor Who audioplays and think they are all of superior quality, but you've really got to forget about that when you listen to this.
This still in no way means this is bad, quite the contrary it's great fun to listen to, but you have to remember that it was based on a play that was rather light hearted in the first place. It's obviouly Doctor Who on it's most 'lets make something fun for the kids' level and the plot is therefor not very complicated. I'll admite it does have some tacky songs, but it's also got romance, adventure, space mercenaries, Cybermen and Daleks, so you just can't go wrong really.
It's probably not a story I'd listen to over and over again, but it is worth putting in any Doctor Who collection, especially if, like me, you regret the fact that you couldn't see it on stage in the first place.