The Casual Vacancy is an attempt to write a sort of top-to-bottom, state of the nation novel using a small community as a proxy for society as a whole. You have a small rural town which finds itself unwillingly attached to a more run down housing estate, and the main focus of the novel is around the untimely death of a local councilor, which opens up a seat on the divided council at a crucial time in a petty, long-running debate over the relationship between the two. The narrative switches freely between semi-internal narratives of a sprawling cast, covering both sides of this divide and a host of people who are affected by it even though they don't really care. It's also loaded with social commentary and more than a little barbed skewering of Middle England in general.
I find Rowling an accessible and fun writer who got noticeably more elegant and complex as the Potter series went on, and ended up noticeably constrained by the heroic narrative she was writing. Here she is freer to let rip, and one of the chief delights of the book is the characterisation; the small mindedness and the willful blindness of most of its characters playing well of the rapid switches of perception. The first act of the novel serves to set up the tapestry of the town, and isn't really kind to any of the characters, although I felt she has a lot more sympathy for the teenagers (even the asshole ones) than any of the adults, and the slightly heavy-handed setup of the Weedon family, whilst effective, telegraphs a little too much the eventual direction of the story.
By about the half-way mark I was starting to get worried however, as the freshness of the structure wears off and the story grinds the gears a little as it accelerates. It's hard to pinpoint quite why, but I think its the shift between the unfocused world-building of the first third and the more focused, building-to-tragedy final third doesn't really manage to find the strengths of either. Once it's over that hump, however, it runs pretty smoothly to bring everyone together for a slightly artificial, yet effective finale.
Overall its a pretty good book - it's got a lot of heart, some good characterisation and it's a bold attempt to write in an almost Dickensian style novel in the modern day. It's also a pretty quick and friendly read, so well worth your time, in my opinion.