I just installed a duplexer on my HP Deskjet 5650, and so far, so good.
I've only done a few test pages so far, but they all worked as they were
supposed to. All I had to do was turn on duplex printing in the HP
Device Manager. What could be simpler?

The pause for the first side to dry was much longer than it needed to
be, but there doesn't seem to be any way to change it. Other than that,
I'm very happy.