In that something like 80% of this book is dialogue, and there is very little description (we don't really know what Colin or Mandy look like, for example), it is quite quirky, and at times not all that clear to read. However, it is curiously engaging in that 2 or 3 days after reading it, you catch yourself thinking about the characters. There are, as Nick Hornby points out, some cracking one liners, but it isn't really a laugh-out-loud book, more a creeps-up-on-you-quietly story. There's an interesting twist, and Colin in particular is portrayed quite well; the female characters are weaker, and some threads could do with a little more development rather than leaving it up to the dialogue to carry everything. According to the dust jacket, its already been optioned for a film - probably would make the transition quite well.
Overall, then, enticing enough to keep you reading to the end, but perhaps a little lacking in narrative. It'll need another reading before I really make up my mind!