Short:
These are the rarest of children's books - ones that you can enjoy as a child, and then love again as an adult. Pratchett's wit made me laugh at 10, and at 21 I can still find them funny and moving, perhaps even more so. Lovers of Discworld or Good Omens will find a lot to love about the descriptions and situations in the Johnny books. I'd say they're a perfect introduction to Pratchett. They're inoffensive without being soft, and smart without being difficult for children to understand.
Long:
The Johnny books are, first and foremost, fun to read. Pratchett's descriptions and dialogue are funny and colourful as always, though his use of footnotes and paragraph long metaphors are less in evidence than usual. The plots in each of the books are tight, touch on issues below the surface of what's happening, and are never needlessly dumbed down for the audience. The last point is something I'll return to later in this review.
Johnny is very easy to relate to - a sort of pubescent everyboy with an overactive imagination and trouble at home. His friends are painted with equal skill and humour: Yo-less is a black kid, so named because he isn't cool and he doesn't say "yo*"; Kirsty is a voracious overachiever and is constantly in exasperation with the boys, I am still in love with her; Big Mac is a clueless skinhead who has an inexplicable talent for mathematics; and Wobbler is such a nerd that he WISHES he were a nerd.
These divergent personalities interact and play off each other delightfully, and they have far greater depth than most characters in young fiction. Most importantly, none of them are stereotypes, and all of them are sympathetic.
The books aren't just rereadable because you'll love the characters. You'll find themes that go beyond the surface plot. Only You Can Save Mankind has an important message about respect for life, whether it's human or not, or even real or not. The other books carry their own themes, but I'll let you (or your children) discover them!
Adults will notice that real events of the time parallel with the story, and a great deal of subtextual information is there for the perceptive reader. Pratchett has pulled off a very difficult balancing trick here: there's plenty of adventure and comedy, but there's substance as well.
All in all, I can't wait till my nephew is old enough to read these books, or for me to read them to him...if only for the excuse to read them again!
*Remember these books were written in in the early/mid 90s, when it was wicked to say wicked, and using "yo" as a greeting was assumed to be the province of black kids.