Yeah, if you're talking about the fonts from the exljbris foundry, yes, they rock! And (at least for the ones I didn't take from previous hacks) I only used the freely available styles. (And generated crude bold/italic style if they were mising via FontForge). I'm not sure if it's actually completely following the licence, but it is better than simply pirating the commercial full family. (Because If we're going that road, technically Caecilia (the default Serif font) is a commercial font owned by Adobe... But since it's provided with the Kindle, and I'm assuming people who are here actually *own* a Kindle, it's more or less okay...).
@brecklundin: I use Serif_* to encompass the four styles (Regular, Bold, Italic, BoldItalic, since you usually use the same family for all four styles). But it *is* possible to use different families, provided they have more or less on the same scale, and the same character length/spacing. For exemple, I'm currently using Museo Slab for the Regular & Bold styles, and Caecilia for the Italic & BoldItalic styles.
(And the full Museo Sans for the Sans_*)