We enjoyed this one, for the most part - obviously, my 7yo had a better time than I did, but I still find these books less of a chore to read aloud than some children's lit we've tried. There were some accuracy issues in here that made me cringe (trying to let that go, but not quite succeeding), but it's fine if you are reading solely for entertainment. I think Geronimo Stilton has come to stay at our house, at least for a while, but there are worse books series for the kiddo to get hooked on.
While the information is occasionally dodgy in this, and I have my reservations about corporate assembly-line literature such as this, my son did find it very entertaining. He's not a "reluctant reader," at all, but it's still nice to put fun books in his path, especially during summer vacation. Plus, these books have become available in basically every language, now - that will be a significant boon when he becomes ready to practice reading in languages other than English. The biggest drawback for me is cousin Trap. I loathe that guy.
So, "audition for the family library" has been successful, though very few of these books will ever actually make the physical family library shelf. This series has way too many volumes for us to bother buying into it; however, thanks to the public library, we will likely spend many years with Geronimo Stilton, and I can live with that.