I don't have the book in front of me (nor have I read it in a while), so I'll have to keep my options rather general...
It's a very interesting concept (one who often uses performance as a way to expose the faults of others), but as far as my campaign goes, I can't see players getting much use out of it. Of the various Power Classes, I'd put the Fool in the same category as the Hedge Wizard...a class that I would use for a special NPC or maybe a one-shot player character, but just not a class that could serve a long-term role in a party.
(If you want my point of reference on what I DO consider really useful, I expect both Noble and Explorer to be core player classes in my next campaign.)
The Fool has low hit points, few weapon proficiencies, and no spells to cast, so he/she is definitely not a combat monster. Instead, each Fool focuses on an area of expertise, such as physical performance, interaction, subterfuge, and so on. Physical skills such as acrobatics and knife throwing could be useful in a pinch, while some of the interaction skills are useful for gathering information. Really skilled Fools can even pass themselves off as "prophets," using supposed visions of the future to convince others to do what they want.
However, some of the other skills seem less helpful. For example, a Fool skilled in certain aspects of interaction can taunt others into attacking, but I don't believe the skill actually causes the target to suffer any penalties for doing so. So, unless the Fool is luring the target into a trap set by others, just why would he/she want to incite such anger??? Insulting an dungeon-dwelling ogre when you don't have any serious combat skills seems like a poor idea, while insulting people in more normal environments (like a town) would probably only be useful in limited situations.
To summarize, I would say the book is useful for NPCs and possibly very roleplaying-oriented PCs, but it just won't serve any use in a combat-heavy campaign. Its not at all a bad book, mind you...just one that fills a relatively small gap in the class landscape.
P.S. It's late, so I hope some of this vaguely makes sense!
