I can't say how widespread the usage is yet -- it's a new e-reader and FB2 originated in Russia, so it still needs to set its roots in the West.
But the spread of the Nokia 770 will, I think, bring it to a position of significance. I speak, of course, as an adherent and an optimist. But when you compare its features to those of the Sony Reader, I think it will benefit from Sony's push into the e-reader area. (See for instance pieces I wrote at Teleread, "Nokia 770 compared to dedicated e-readers,
http://www.teleread.org/blog/?p=3994, and "Sony's ereader: the good news and the bad news,"
http://www.teleread.org/blog/?p=4058, and many earlier pieces about FBReader on the 770.)
FBReader has really great support for formatting and styles. Here's a screen capture showing italic within text --
This is from one of two pages full of screen captures showing its features and controls, at
http://only.mawhrin.net/fbreader/maemo/screenshots.html and
http://only.mawhrin.net/fbreader/maemo/fonts.html