I am so glad I stumbled on this thread. Wanting to sync reading positions between two Android devices and two PCs. Originally I had used FBReader on Android to read. But in order to sync it I would have to root my nexus 7 tablet as it wasn't rooted yet. Which would mean I would have to backup everything, as rooting destroys all loaded apps. Also it wouldn't sync to PC of course. I then started to check out many other readers on Google Play. I also tried the Kindle app, but I couldn't get it to sync at all.

Now after a week of fiddling, I stumbled on this thread and tried Google Books. At a first try it seemed to sync, between one PC and the phone. Very cool! Will try to switch devices the next few days and see what happens.

BTW I have the calibre instances of those two PCs and another two (one of them linux!) synced with Dropbox. Ideally the reading solution would make use of this, somehow keeping the reading positions stored in the library itself, but hey, this is quite ok for the moment.