I
loathe Caecilia as a body font. I associate slab serif with headlines and signage, not reading. And Caecilia is particularly round, so I also think it is a "happy" font, but more like Courier than Comic Sans. Comic Sans is an abomination that doesn't deserve to exist, IMHO.
Caeciia Condensed looks like it might be marginally better, but I'm reserving judgement until I see a Kindle at Target or Staples.
I suspect Amazon went with slab serif fonts because they have nice crisp lines, which probably display stronger on E-Ink.
But until the Font Hack is working on Kindle 3, I won't even give it a look. My Cybook can use any font, and I love reading with Arno Pro or Georgia. I prefer serifs for blocks of text.