I still have the books; I ran across them when we moved recently and realized that I need to look for hardcover editions. They are all in bad shape, despite attempts at mending them over the years.
The books are still funny, and they still develop the important critical and analytical thinking skills needed to imagine a different time, place, and way of life.
I think that the books are well suited to the 9-to-12 age range that is usually recommended. Each chapter is typically a separate story, which makes the series ideal for the reader who struggles with longer works. They're officially "boys' books," but the publisher's notion of the primary market segment didn't stop me or my sisters from enjoying them.
(A slightly technical aside: what makes a publisher think of a book as a "boy's book" or a "girl's book" is not just the gender of the main characters (which are all boys here). It's also the style: boys usually buy action-oriented books, and girls usually buy books which invest more time in thought, dialogue, and emotion.
(To give one example of this phenomenon, Bridge to Terabithia, whose main character is a boy, is definitely a girl's book.
(While the Great Brain stories are almost always action-driven, there is a fair bit of thought analysis in these books, so I think they're a little closer to the midpoint of the gender spectrum than the typical "boy's book."
(For example, something will happen, JD (the narrator and the Great Brain's younger brother) will then analyze the situation: if I do this, my friends will think this, Papa will think this, and Mama will think this. If I do this other thing, then everyone except Mama will react this way. So I will do this other thing, and be prepared to give Mama this excuse for what I'm doing.)
I want to add for the sake of those who might have to listen to complaints (e.g., those on school library committees) that various bias and sensitivity organizations which review children's literature have occasionally rated books in this series as racist, sexist, materialist, individualist, and conformist.
That is, the various stories (many of which are at least semi-autobiographical) in the series depict things like:
* the horrible treatment of a new Greek immigrant boy at the hands of the town bully (as well as the silent compliance of most of the boys -- isn't that just like real life?), and the town people's shocking indifference to the welfare of an elderly Jewish man (thus the racist label: people behaved badly at times in the books -- just like they do in real life -- except that here, they're all sorry for it in the end);
* fairly run-of-the-mill girls-have-cooties themes and some historically accurate gender roles (and some historically accurate breaking of gender roles: for example, Mama is a crack shot with a rifle) (thus the sexist label);
* The Great Brain himself is tormented by his Money-Loving Heart and constantly scheming to get more money (thus the materialist label);
* The Great Brain has a me-first attitude (which is partly balanced by actions like whipping the bully for mistreating the immigrant boy) (thus this individualist label); and
* the boys in the story have an informal code of honor (you don't pick on kids younger than yourself, for example) that they all conform to, and social censure is applied by the whole group (none of the other boys will play with you any more if you break the code of honor) (thus the conformist label).
I was thinking of their code of honor the other day when I took a couple of neighborhood girls to the park and witnessed a (completely unsupervised) boy of about eight or ten annoying a couple of preschoolers. It sure made me wish for a time when boys thought that annoying little kids was dishonorable.)
Over all, I think these are excellent books which every child should have the opportunity to read -- and I'm searching now for a complete set of the books in hardback editions.