I just did this in Tokyo a few weeks ago. They did want to see evidence of onward travel and the wait was about an hour, but still far better than sending my passport away. No problem with the add-on pages, they fit into the passport just fine but are a slightly heavier weight of paper than the rest of the passport. They're the new "patriotic" pages with pictures of soaring eagles and amber waves of grain and such.
Upon returning to the US something definitely popped up on the computer at immigration and I got a little bit of extra questioning about where I had been and some serious typing into what I assume is my permanent record that will follow me around the rest of my life. I suspect this was due to the passport being amended during the trip.