My phone is still just as responsive but while playing with it I've experienced the following problems so far:
1. While playing with Cortana I tried random searches. The last thing I tried was Nasdaq which it got right where it showed the search text but then said my music queue was empty instead of anything related to stocks. Now when I search for it nothing happens, just starts over asking me to search for something.
2. While playing with setting images behind the live tiles, it was working and showing me the one I selected but then started showing me the wrong images. I kept picking an image and it would show me a random one I had picked previously. I rebooted and now it's showing me the right one.
3. I tried to shutdown the phone and it reboot instead.
I have a Lumia 928. I'm not complaining or anything, glad to have the update.