I had the same problem with my 650. For some reason the userstyle.css just wouldn't work for me.
What I ended up doing was changing
Code:
font-family: -ua-default;
to
Code:
font-family: DroidSerif
and below the @font-face rules, added
Code:
body {font-family: DroidSerif; line-height: 1.2em}
You could (maybe even should...) use
font-family: serif; instead, to catch cases where the epub file specifies a serif font.
However, if someone tells me how I can get
-ua-default working, I'd love to know.