I've been tested this over a few files so far, and it seems to display them all fine (including the LilyPond scores, which have caused some readers grief in the past), with a nice quick startup (which cannot be said for Adobe), relatively smooth navigation (unlike a few other free ones), and a single file app (I've always said that apps should ideally be single-file like this... portable and compact).
Heck, it's even GPL'd.
The one downside I found is that it's default startup screen has a really garish yellow colour that sears your eyeballs if you sit there staring at it for too long before loading anything. Fortunately, to nuke it, just start it from the command-line with a line like
SumatraPdf -bg-color 0xffffff
which will set the background colour to white (or whatever hexcode you like to throw at it), storing this setting for the next time that you start up too. Luckily this was quite easy to find from the manual.
The other downside I guess is that it only runs on Windows. Then again, Linux has its fair share of nice browsers too, so this doesn't really matter for those of you on that platform :)