I've had decent success with a Kindle device, but when I try to use the Kindle app on my iPhone, which is rare now, it's almost always a hassle. Their iPhone app updates completely replace the app, so everything gets reset and books have to be downloaded again.

But the main problem is that they don't sync the "last read" bookmarks until you open a book. But since that book didn't have a bookmark, it's reset to the beginning and then synced, so my "last read" bookmark is now at the beginning.