I'm going to use the Noble and the Explorer as core classes in my next campaign, and I'm curious to see what the Fool will be all about.
Gladiator, Knight, and (probably) Pirate strike me as classes that almost need your setting to be tailored to them, and I just don't see such "types" occurring much in my campaign. I have my own assassin variation in the works, and the rest of the classes fall into the "interesting but I have no immediate use for them" category.
However, despite my current ratio of using only two out of the eight published Power Classes, I do find the whole series interesting. Mongoose: Keep 'em coming, and I'll keep picking 'em up!