The Gates is the story of young Samuel Johnson, an endearingly odd boy who tries to show initiative by trick-or-treating three days early, and who brings a pin to show-and-tell under the auspices that one cannot prove that there aren't angles dancing on its head. Samuel is a winning protagonist, and his constant companion, Boswell, is one of the most expressive non-talking dogs I've ever had the pleasure to read. Unfortunately, Samuel's adventure, which begins when he sees his neighbor's satanic ritual and it's unfortunate results (they open a portal to the eponymous gates of hell), is regularly interrupted by clever footnotes and charmingly informative, though plot-killing chapters on physics, black holes, and other assorted items of a scientific nature. Granted, a Large Hadron Collider helps open the gates of hell, so a little background information is useful. The information is also presented in a super-charming way, but it's that very charm that distracts the reader from the actual goings-on of the plot.
The Charm (tm) that pervades The Gates is one of those qualities that is fantastically effective in the proper dosage. Terry Pratchett, more often than not, manages to deploy a similar kind of charm to good effect in his Discworld series (though, sometimes, he too hops over the charm-line into the realm of the self-conscious and precious). Connolly slaps Charm (tm) all over everything, cluttering up his prose and wearing the reader down so that it's hard to appreciate it when it's appropriate.
Disclosure: Connolly's particular brand of charm involves the frequent use of something which is an irrational pet peeve of mine. I hate it when an author does this: "She was about to jump off the cliff but, well, she didn't." I really hate that well. Connolly uses it, well, a lot. It might be okay in speech (although I don't like it much there either), but dialogue and narration are not speech, they are the written representation of speech. As such, they leave no room for um's, huh's well's and the like - especially not when used with compulsive frequency. It's just not, well, cute.
That said, The Gates was good fun, and I honestly liked Samuel Johnson. He's a believable boy, despite his carefully chosen idiosyncrasies, and for that alone I think The Gates is worth reading. I would just consider borrowing it from the library instead of, well, paying full price for the hardcover :-)