FWIW, and in brief because Android just killed a longish post. My LAN has 4 Supermicros, 3 ASRocks, and one 2xL5639 Lenovo D20 board in a Norco chassis, plus I built for temporary media playback, a B85M-ITX+G3320. In fact, I would say positive experience on that board which I bought because it was cheapest and not ECS - like VT-d support, and pulling NTP servers and offset from DHCP (why Windows 8.*/2012* insists on GMT-8 when my BIOS can figure it out and the DHCP server is a WS2012r2 machine is ridiculous) helped get me over my reluctance, and I'm glad for it.
I have had silly design problems with Supermicro - not usually reliability ones, barring one stupid EFI shell text overflow - and fewer with ASRock. As far as just working/reliability, in a whitebox/self supported environment, I don't really care much one way or the other, though I think that ASRock certainly is making an effort in white box/lab building, which is just plain smart. I also distinctly prefer ASRock IPMI, both web and Java interfaces, and despise Super Doctor.
On point, though not stress tested (just learning FreeNAS, also I'm waiting on a 4th Travelstar 1.5TB 2" to go RAIDZ2) I did buy the ASRock C2750 for NAS use, maybe also as a low power Windows LAN client with a mirror/data store dual use. Install was done by IPMI with remote ISO mounting with no issues.
Also, I think 204 vs 240 pin matters, or it certainly does to me: if/when 16GB UDIMMs become available, I can pull whatever UDIMMs are in the C2750 now, step back, and look at all my DDR3 machines (except the 2011s which are already on 16GB sticks) and figure out how best to distribute them. I would say there's a lot more reuse value in an 8GB ECC UDIMM when in a 240 pin format than 204.