After doing a lot research about compatible CPUs in all forms of the various Mac Pros (I own several models), this particular combo came up very frequently. Almost all said it wouldn't work, but I couldn't find PROOF that anyone tried it and failed (well... there was ONE old thread I came across where it didn't work, but details were sketchy). So I looked up the price of a used one on eBay... it ran between $45 and $50 used. Not too bad, and certainly not expensive enough for me NOT to try it. So I made the purchase, and after a few days, it arrived on my doorstep. Now then, about my machine. Its a single CPU, early 2009 Mac Pro 4,1 (flashed to 5,1) that I swapped a X5690 into. Its equipped with 12GB (3 x 4GB) of DDR3 PC10600R RAM running at 1333mhz. OS is on a PNY CS900 120GB SSD in drive bay #1. Not too shabby, but really it's a basic setup. I popped the tray out, removed the heatsink and processor, and cleaned everything up. Plopped in the X5687 CPU, added some thermal paste, screwed the heatsink back on, and slid the tray back in. Powered it on and crossed my fingers... The machine turned on, sounded like it was coming to life, but no chime and I was treated to a black screen. So I powered down, took out the CPU tray, looked everything over, and re-inserted the tray. Powered back on, and still nothing. Feeling like perhaps I did something wrong, I pulled the X5687 and checked for anything out of the ordinary. Nothing looked out of place, so I installed the known working X5690 and fired the machine back up. Power on, subtle noises were heard, got the chime and boot screen. Everything fired back up and was working 100%. In conclusion, it seems as though the X5687 officially will not work... well at least in a single CPU, flashed 4,1. I'm probably going out on a limb here, but maybe in an actual 5,1... or perhaps in a dual CPU setup... results would be different. There also is always the possibility that I got a DOA chip... but sadly, I have no other system to install it into to verify. So at least for me and my setup, I can say without a doubt, the X5687 is not compatible. Hope this helps anyone that was still on the fence or had additional doubts.