Here's my completed build (pics taken with crappy tablet camera):
-the Noctua NH-L12 is compatible with the Maximus VI Impact. However, if you orient the cooler as I have, the front audio panel header on the sound card daughterboard becomes useless - one of the heatpipes stands in the way making connection impossible. Quite a stupid design decision to have the connector facing the crowded CPU socket area. You can orient the cooler with the heatpipes facing towards the front or the back of the case but this way the bottom 92mm fan is unusable and temps are generally higher due to the concave surface of the bottom of the heatsink.
-Noctua's fans are quiet but since the 4771 runs very hot I decided to replace them with more powerful ones: a Scythe Kama Flex 92mm fan and a Coolermaster Sickleflow 120mm.
-the front intake fan is very quiet but draws in almost zero air, I'm going to have to replace it in the future. For exhaust I'm using a high static pressure Corsair fan right next to the motherboard.
-cable management is passable, although Corsair could have included more hooks throughout the chassis for better results. The Elite 130 was a lot better in this regard.
-the case feels very sturdy and panel fit is almost perfect.
-Asus uses very cheap pigtail to RP-SMA cables. One of them broke while transferring the motherboard from one case to the next. Also, the included Broadcom 802.11ac mini pci card, while supposedly fast, just doesn't work with my 802.11n home router. The connection is incredibly flaky and driver updates haven't helped. According to various users at Asus' forums this is a know problem with non-ac routers. I've never had much success with non-Intel network cards - I've ordered a 7260 to replace the Broadcom chip.
-temps overall are lower with the Obsidian compared to the Elite 130.
-this case, despite the high rpm fans, is quieter overall, by a big margin.