It's as good as you'd expect, probably moreso.
I hadn't ever heard a Who audiobook before so every aspect of it was something of a treat. The Doctors and their companions haven't aged much, in terms of their voices, they sound roughly as they did back in the day. Each of the five have their spotlight moments, and are also teamed up with one other of the Doctors for periods of the story.
Briggs has captured the personas brilliantly. The digs between the Fourth and Eighth Doctors are reminscent of Troughton versus Pertwee. The Fifth Doctor's curious approach and his somewhat hapless inquiry of the poor family man in 1963 before finding a solution are typical. The Seventh Doctor's preposterous language enlivens, and he is somehow an 'elder statesman' of the group. The Sixth Doctor's 'immodest' presentation has me liking him more and more as I get older-- his dressing down of one of the enemies is priceless.
When it was done, I found myself wanting this to go on for another hour!