Read this aloud to my 11 and 10 year olds and it worked surprisingly well for that age. (Had to do far less verbal "editing" than with Ender's Game, for example. If this was a movie it'd be a solid PG.) The first chapter was funnier than I remembered, and the next quarter of a book was less funny than I remembered -- it's hard to make light of the destruction of the Earth when you're reading out loud to children -- and then once we got to the Babel Fish it picked up again. My favorite part still must involve the sperm whale and pot of petunias, which even made it into the movie.
If you want evidence for how the Dawkins/Adams/Doctor Who crowd is a mirror of the Young-Earth/Ken Ham crowd, this little book actually provides it. There's even a literal young earth involved in the plot! Also, bits about the proof of God show that the reasonings of the two groups run in parallel. The difference is that Adams is intentially hilarious, and he never takes himself too seriously.
When I was in high school and still of the young earth persuasion, this book actually didn't challenge me at all, and may have even reinforced my erroneous belief that old earth implies an disordered and irrational universe. The humor that bothers me the most is the humor that verges on nihilistic and pessimistic, not the humor that takes on religion. But when it's all taken lightly enough -- when you fall and forget to hit the ground, as from later books in this "trilogy" -- it works overall and may turn into one of the best uses of humor. And there's something kenotic about that.
In fact, I would argue that there's more depth and compassion in this book than in all of Dawkins' writing. Behind all the truck about probabilities and irrationality is a yearning for the certain and rational, wrapped up in a human narrative. The theme of falling and humanity that runs throughout the trilogy is one example of a strand that could provide some interesting literary analysis. But no time for that -- for today, I just enjoyed having a laugh with my boys.