Well I eventually went, and actually saw a pretty good film. It stars Casey Affleck, Morgan Freeman and Ed Harris in a film that sees a young child get kidnapped. Affleck (a private detective), Freeman (a police chief) and Harris (a detective working the case) called in to solve the case.
The film did pretty well in its' attempt to maintain the general thrust of the book (although I think they might have changed the ending) and it caught Boston pretty well. I also applaud it for laying the dark underbelly of the city bare.
I do have a few grumbles though. Firstly, Afleck's partner in the film is severely underwritten. She's just this sounding board for Afleck's ideas in the film, and I don't think that works all that well. Secondly, I think Lehane writes conversations between friends well. There's a certain amount of dark humor in the books that doesn't get translated to the screen too well because Afleck doesn't get to use it on someone. Thirdly I've interned in Boston over 3 summers and I found the "blue collar accent" unintelligible in places. I don't mind it but it might irritate some people. Finally it's kind of depressing. Don't get me wrong, I like depressing, bit it might explain why it didn't do to well in the cinema.
All this having been said, it's still one of my five favorite films of the year and would recommend people go and see it (which they probably won't given how long it's been out).