Just wanted to add my experiences to this thread.
Bought a 27" i5 iMac in april. I also bought a 16 GB Crucial memory kit. Brought it to my office and set about installing my applications. Had the first reboot after about two hours using it. No error messages or kernel panic; just the full on sudden reboot everyone else seems to be reporting.
Since I had installed third party memory, my first notion was that the memory was the culprit. Removed the additional memory, but still the machine would boot one or two times per day while in use, and always during the night while it was not in use. (I always let my office mac run during the night to be able to remotely access it via remote desktop. My old Mac Pro has been running like this since I bought it in 2007, and I never had any problems with stability.)
After reading a little online, I still suspected memory trouble, and rebooted to Apple Diagnostics to test memory. The memory passed with no problems. I then downloaded a Memtest86 USB image and made a bootable USB to check for RAM problems. It found > 1000 errors on test 10. Downloaded the newest beta version 7 of Memtest86 as well, and ran that from USB; same error. Loads of errors on test 10.
Contacted the retailer where I bought the machine, and they took it in to diagnose it. They also downloaded Memtest86 and ran the test, but found no problems. They had the machine running on an external drive for six days without any problems whatsoever. I then gave them access to my installation, and sure enough; the night after I let them in to the installed OS X on the Fusion Drive, the machine rebooted during night as I was experiencing.
The tech read the threads here about people having trouble after upgrading to 10.11.4 and suggested that he'd wipe the internal harddrive and repartition it from scratch, installing 10.11.3. I got the machine back this monday and haven't had any issues since. Seeing that quite a few people seems tp have had stable periods that last a week or two between the crashes, I'm still not sure it has been fixed permanently, but it for sure is a lot more stable than it was on 10.11.4 or 10.11.5 where it would restart at least three times per day.
I find it very interesting that Apple seems to be replacing hardware to remedy the issue, as a lot of users are claiming the trouble only started after a software upgrade. A botched firmware upgrade might explain the issue, but my iMac came new in the box with 10.11.4, which means that there was no upgrade done in my case.
I am looking forward to see the solution to this and see what is really causing the trouble.
-KJ