Thanx! Hope this concept will succeed!
Bad luck again though...
I've gotten an Gigabyte GA-Z77N 1155 motherboard from Germany last week.
Unfortunately the seller, even though I asked for proper transport protection, simply put the board in a box with plastic bubbles around it. The package looked beaten a lot when I got it, and half of the bubbles protection where flat.
I didn't take long to notice a bent pin in the CPU socket : /

So I carefully put the pin back in place the best I could, and to my surprise the board reached the BIOS once powered on!
That was with the integrated graphics though, and alas, no way to use the PCIe slot. The 3 GPUs I tried are not detected.
After asking the Gigabyte tech support, which unlike Asrock did answer me, this problem can indeed be related to the bent pin.
I will try to improve my fix, but I' afraid this board is going to be just a waste of time and money (150EUR)...