I saw a Nook (classic) at Walmart on Sunday and was blown away by how white the background looked and how beautiful the font (Amassis) looked. This after 3 weeks of reading on the PRS 350. So, yesterday I spent the whole day reading about fonts and spent the evening experimenting with the 350. I also looked at my friend's Kindle for the Caecilia font.
One of the things that Kindle does well is that it uses the same font consistently. My 350 uses so many different fonts that it seems annoying.
With that in mind, I decided to modify the userStyle.css (I have Baroda firmware) to point all the other known fonts to CharisSIL. I chose Charis because a lot of ePubs use this font embedded and it looked most Kindle like on my 350. So if many ePubs embed it and my 350 uses it for those that don't, I would see it everywhere, right? Well, Calibre created news sources use a variety of fonts. Then of course, my PDF's use different fonts too. There is nothing I can do about the PDF's but for news, I decided to use the "font misdirection" method. I substituted Georgia, Verdana, Times, TNR, and serif with Charis.ttf resource. I also substituted Arial, Helvetica and sans-serif with the same. It is giving me some formatting issues and I need to experiment some more today. But the idea seems to be working.
I would then create another userStyle.css file that points to a sans-serif font for everything the same way. This will allow us to switch fonts like on Kindle or Nook. And it will apply them consistently.
So far I have compared Charis with Droid, Lexia DaMa and Constantia. I like Charis and Constantia a lot. Charis is a bit chunky and looks good but Constantia is easier on the eye while reading and looks generally more pleasing.
Edit 2/16/2011:
It worked even better than I expected. I tried it with Fontin font and it replaced the font in ePubs embedded with Charis too.
Here is a link to my epub folder's zip
http://dl.dropbox.com/u/8856395/epub.zip
Unzip it and replace your epub folder to use Charis font everywhere. Pick another file and rename it to userStyle.css to use that font. Of course, first you will have to rename the current userStyle.css to userStyle.Charis.all.css.