Just finished listening!
I thought it was basically OK. There wasn't much that I hadn't read in the book already, so a new listener might find it more original. The music was the original classic intro song by the Eagles, and some new orchestral stuff that sounded too good, i.e. it didn't fit with the rough low-budget 80s feel of the original. However, I don't blame the producers for it not being the 80s any more, and besides, the new ambient-style music at the beginning was excellent. The origins-of-the-screwdriver story was set to music in the style of the original theme tune which sounded suspiciously like Sing by Travis, which may have been unavoidable; there are only so many things you can do with a banjo...
I'm not sure if I liked them alternating between the new The Book and the old one. Seemed gimmicky, as did most of the playing around with the audio. I don't listen to radio at all, so maybe audio special effects are just par for the course in this medium.
It also suffered from the same thing all sequels to beloved works suffer from: pauses. It happened with Red Dwarf in series 7 and 8. The lines are delivered more clearly and slowly, seeming like they really wanted to make sure you heard the joke.
Good bits: the explanation of Wowbagger's prolongment was perfect. "Oh dear... Oops... Oooh." Marvin was marvellous, naturally, as were the mattress sound effects.
Bad bits: Was it me or did they imply Zaphod stole the Heart of Gold for the pure Gold at its Heart rather than simply because it was "so amazingly amazing I think I'd like to steal it"?. Also, the opening sequence with Arthur seemed kind of flat, but again that may be because I knew it all from the book already.
Conclusion: 8/10. They'd've had to really screw up to make it bad, given such awesome source material.