Personally, I've been disappointed with every Quint book. Most of the prestige classes, feats, and other crunchy bits are often poorly balanced. Some people complain that WOTC's splatbooks are unbalanced, but those people must not have seen the Quint books. (For example, the Quint Witch is incredibly overpowered, being a divine (thus, can wear armor) spellcaster with almost all of the best wizard spells in addition to cures, and then you can choose to not gain the "long life" abilities (if you are hunted) in order to start with Alertness and *Evasion*.)
The Quint books might have more flavor. *Shrug*. All I know is that they've got more naked titties.
Plus, my impression is that the authors of the Quint books don't quite have the same understanding of the 3e rules as the WOTC writers do. (For example, the Quint Witch spell list includes spells that don't exist in 3e, but which did exist in 2nd edition.)
And of course, there's the matter of value. In the games I play, the DMs allow anything from an "official" source, so I know I can use anything from WOTC's books, but if I find something I like in a Quint book, there's a very good chance I won't be able to actually use it.