Fight Club is a very interesting movie, but I'm not going to review the movie, because that has been reviewed here hundreds of time.
The bluray: before I bought this, I was doubtful whether it would matter to have this movie on bluray. I can say I am satisfied with this HD version. The sound is very clear and the music in this film (in the beginning for example) seems spiced up compared to the dvd version and is cleary superior. The picture quality is robust: this movie had some soft grain, but I think that it was the intention of the moviemaker. Somehow, in this movie, it adds character. There's also an interesting depth to the picture which I cant recall from the dvd. Furthermore, a lot of extra detail can be seen when watching this on blu ray. There are some documentaries on the disc, but I didnt watch any of them so I cant comment on them.
In short: if you like this movie, and dont own it yet on dvd (I didnt own it yet), you should definitely buy the blu ray, its worth the extra money because it sounds (a lot) better and it looks better (and grittier). If you already own the dvd, it depends on how much you like this movie. I wouldnt buy it again if that were the case.