When I first set mine up, I had it restore a backup of apps from my GS4. Not sure if that was a good idea or not because I do have some instability issues:
Wi-Fi regularly turns itself OFF when it should be On. Usually it reports an authentication error with my Asus AP, but if I have it connect again it works fine. Never had problems with this Wi-Fi on my GS4. The nexus doesn't seem to retry connecting.
Also, the phone has randomly crashed three times since I got it. Once after having it less than 24 hours, the rest after 2+ days of use. As part of these crashes I have found that the annoying AT&T startup sound always plays, even when my phone is set to silent mode and also at max volume. Not ideal if it crashes at a time when loud noises are a bad thing.
Last issue is audio quality problems with Bluetooth streaming. It sounds like the audio is playing from the other side of a spinning fan blade. If I turn off BT and then turn it back on the problem corrects itself. This is rare but has occurred twice in the last 1.5 weeks and I use BT streaming a lot.
On the good side I really do like lollipop and the new UI design. The device hardware is good, battery life has gotten better since the first few days (couldn't make it a full day on my GS4, but easily make it more than a day with the Nexus 6). The screen is beautiful and the device is always very fast/responsive.