I have been having two problems with my rMBP 13", one of which is identical to what many of you guys are experiencing.
Problem 1 (freezing):
(I misspoke and said August, but this happened last month, in April.)
This occurs mostly when I have a lot of cables and peripherals connected and am using external monitors, etc. (because I work at my desk and rarely use my laptop for long periods without an external monitor), although it also occurs without all of that. One quirk I noticed was that if I didn't unplug all of my peripherals when rebooting after a freeze, bluetooth would show in the menu bar as "Not available" and I wouldn't be able to enable it no matter what I did—I had to delete the com.apple.Bluetooth.plist file in /Library/Preferences and reboot with no peripherals attached to enable it again.
Apple replaced the logic board and a trackpad cable last month and this problem has only occurred once since then (within 15 minutes of restoring my Time Machine backup), and the strange bluetooth behavior did not occur at that time. Since that initial instance, the laptop hasn't frozen once. This could be pure coincidence (I've gone for long periods of time without issue before) or there could have been an underlying hardware issue in addition to whatever may possibly be wrong with OS X 10.11.4.
In any case, I'm glad they replaced the logic board, as that is at least one more potential problem fixed.
Problem 2 (probably unrelated to this thread):
Beach ball of death. A program would seem to stop responding, and shortly thereafter every program would stop responding, followed by the dock and menu bar failing to respond, and eventually I'd just have to power off the machine using the power button. Leaving the machine on in the hope that it would eventually recover somehow resulted in no change. Using force quit and attempting to kill each program one by one also did not work. Happened before and continued to happen after logic board replacement. Seems to have little or no relationship with system load.
I disabled Flash in Chrome and as far as I can recall this beachball of death thing has stopped, but since it's a relatively infrequent problem, it's difficult to be sure.
I've never had a computer without hardware problems crash this much, and that includes ancient Windows machines. It's tough to try to work around the constant possibility of a total system freeze. Let's hope they get this fixed soon—otherwise, this might be my first and last new Mac purchase.