My iPhone 6 has been anything but stable, it crash apps at least once a day, I get all sorts of random bugs, issue with WiFi, battery can be amazing one day and horrible the next.
Bugs I get most often are:
* Lock screen just won't let me swipe left (So go to passcode or swipe left on an app), and just gets buggy and very jumpy till I restart.
* Volume meter get stuck until I restart.
* Apps reboot my device once ever few days, and crash at least once a day
* Randomly lose signal and won't return till I restart, tried go airplane mode and back but no luck.
* Handoff can be amazing at times, but at other times I straggle to just get internet tethering to either my iPad or Macbook, and takes me restart everything before it works.
* Generally iCloud bugginess, I get logged out at times form facetime/imessage and so on. No real info on how to fix things too so I end up signing everything out and then back in, sometimes that doesn't fix things.
Not really that stable these days, maybe iOS and iPhones of past where better, and I've already had most of these bugs on 8.3 which I've had installed all of 3 days.