Maggan said:...if many playtesters liked Bo9S a lot, then that's a data point to consider.
Yeah, that's another thing to keep in mind. When I first heard about the Book of Nine Swords, I had no interest in buying it. It sounded like some kind of quasi-East Asian setting book, and I tend to prefer a more European flavor in my games; plus I'm a caster player from away back. I've always been bored by D&D melee classes.
However, one of the other guys in my gaming group is more into Asian-flavored stuff, plus he loves playing melee warriors, so he picked it up and convinced me to take a look at it. I tried out a swordsage and was instantly hooked. I soon bought my own copy and have been a martial adept enthusiast ever since; but if I hadn't had that guy in my group, I never would have bothered with it.
All of which is to say that the success of a supplement like the Bo9S is not determined purely by content. The way the supplement is marketed is also a major factor. If I were WotC, I'd be looking less at raw sales data and more at customer satisfaction ratings among the people who've tried the book.