I'd definitely recommend you research the Book of Nine Swords before buying it. It's certainly not for everyone: the mechanics tend to favor very technical, strategic thinking in building and playing characters, and some people (I'm one of them) think the classes and abilities in the book are significantly more powerful than most of the core material. I'd only suggest it for groups on extreme ends of the powergaming/roleplaying spectrum: either where everyone has a really good time tweaking the rules to produce extremely potent characters, or where everyone is so committed to roleplaying that they'll ignore easily realizable, potent combinations if they think they don't fit thematically (and potentially don't mind being overshadowed by other people in the party).
On the other hand, the PHII has useful, and, for the most part, well-designed material that should be interesting to almost any player--it's particularly good at filling perceived gaps in the system (like a relative lack of feats for experienced characters) and representing classic fantasy archetypes that aren't particularly well modeled in the core rules. It can significantly enhance almost any campaign.