iPod touch (with 4S hardware) here. Some days it doesn't crash at all, some days a lot. A few days ago it crashed three time within only three hours of music playback.
I had my first crash today while on a phone call. My iPhone 5s was locked and I was talking to my wife on the Bluetooth in the car. I needed to unlock the device and used the touch ID and the phone went straight to the apple logo. I was unable to hear here for about 10 seconds and then she was back on. It didn't disconnect the call, just rebooted.
This was the first time it happened and I have had my phone since day one. Reminded me of my last Jailbreak lol.
No system crashes or reboots at all, apps are another story...every day I have apps crash, even Apple apps like the photos app or the camera, but to Apples credit it doesn't crash the system
So no system crashes, but this is the buggiest Apple software update ever, apps crash, apps don't work etc.. big overhaul and in a short time, I guess iOS 8 will be NICE.
Never had kernel panic or reboot on my iPhone 5, but Settings app crashed a couple of time since iOS 7 installation.
Keep in mind that I using it almost constantly to check for the battery drain that seem to be affected my iPhone.