I had some rebooting problems with my T-Mobile unit, but that was back in days post-launch. Once the custom ROMs started dropping, I ran those, but I've been poking around with the 5.1 stock release recently and I have yet to have that thing bounce on me. I remember reading some forums and one of the kernel devs suspected that the storage I/O software was a bit wonky and that the kernel would attempt to read/write to the internal storage before it had fully woke up from deep sleep... and this is what caused the crashes. But that was back in December, if memory serves me right.... there is a fraction of the chatter related to reboots recently. But keep in mind that people with the same issue will often gravitate towards themselves, and this causes a bit of selection bias.
On the whole, the CURRENT iteration of the phone+OS appears to be extremely stable now. If people continue to have issues, they may very well have a defective unit, but the issue is no longer as common as it was.