I just sent in my two GTX 660s. And just got word that they are sending back another 660.
These cards don't seem to want to run in SLI, and will black screen at stock settings under heavy load. Sometimes it takes 10minutes sometimes longer but they crash and only a hard system reset would bring me back. I am unsure what exactly brought this on, however it has made playing games very difficult. In the events leading up to the crashing I noticed extreme FPS drops with the performance permanantly decreased for the remainder of the session. See -
https://www.youtube.com/watch?v=g7B0O9A3EwQ&feature=youtu.be
In that configuration the game should have been way above 144 fps the whole time. As these cards are more than ample for that game at max settings. Even setting the quality levels down during those FPS drops would not make FPS increase leading me to believe the issue was hardware related.
 
I noticed that when those performance drops occurred in Afterburner OSD the GPUS OV Limit would hit 1 (out of a range 0-1) and then the performance would dip or, the gpus would crash.
 
i7 4770k clocked to 4.4ghz (Rock solid stable)
Gskill 2133 8gb ram.
MSI Z87 GD45 SLI
Seasonic X series 750w 80+ GOLD
 
And im not the only one I guess.
http://www.overclock.net/t/1347089/gtx-660-non-ti-sli-crashing
 
https://forums.station.sony.com/ps2/index.php?threads/just-added-second-gtx-660-in-sli-game-now-crashes.119183/
https://answers.yahoo.com/question/index?qid=20140220120125AAc4MCV
http://forums.evga.com/GTX-660-work-separate-crash-SLI-m2211909.aspx#2227170
 
I have trouble shooted all my parts individually, tried my friends gtx 780's in SLI (OC) and got no such issues. I want to know what to do when I get these cards back. 
I read through this on OCN http://www.overclock.net/t/1328184/solved-rsod-when-sli-enabled-680-4gb  And I actually tried the fix on page 7 myself before sending my cards back. It seems to be a firmware problem...
 My friend suggest I upgrade to 760's as they don't seem to have nearly as many SLI issues. Part of this could be because of loss of thorough support through drivers. But like I said im at a loss here.
When sending back cards are they used cards? are they packed in original box? If so I might consider selling them and upgrading.