I'm running firmware version 1.3.1, and I've found now if I add any new fonts, I have to turn the device off and then back on (a full power off and on) before they show correctly.
That aside, I've been really enjoying the Linux Biolinum font lately; it has a very nice weight and straddles a nice line between something formal and something casual. I've been using Severek lately for reading Instapaper epubs; it's a really nice sans-serif but is a paid font. I tried Charis SIL for a little while, and while it displays nicely, it just felt so, so bland and lacking any character whatsoever. I guess depending on what you're looking for that could be a plus, but for me it was actually so generic looking it was distracting.