I picked up the P67 Ext4 Gen3 a week ago Friday with a 2500k at microcenter. I'm using the board as a temp one till my Asus comes back from rma.
The P67 > Z68 jump was too huge for me as it's most likely only a temp board for me. If I was gonna keep the board for the long haul I'd have to think about the jump tho. Z68 features don't look like they are for me.
I've had some BSOD's but overclocking and BSOD's go together
USB3.0 I don't have anything to test it with. Using the drive bay adapter to mount my SSD drive tho. The port is functional but only have tested it with my USB memory sticks.
I'm using the board with my 2700k currently at 4.8ghz without issues. The uEFI is primitive compared to the Asus implementation but has the needed settings to crank up a K chip to be fair.
Overclocking wise my 2700k pretty much clocks the same as it did on the Asus board.
I haven't really found anything negative about the board yet other than the uEFI not being as nice as Asus's.
Newegg reviews can be scary! I suggest taking them with a grain of salt. It's hard to tell if it's a true issue or if it's pilot error.