I just expanded my NAS with these drives a few weeks ago when they went on sale. They're great. I'm actually not sure what people are complaining about noise wise... I found that their noise level compared to WD Reds is actually comparable, maybe slightly louder during seeking but not really appreciably so.
I'm using them in a couple NAS's myself and they've been great to me as well. To date, of the drives I have experience with (limited), these have become my go to. To jgreco's point I have done battle with them a little to keep the drives temps under 40C (while under load) in the Define R5, but nothing that couldn't be overcome.
Yep, run a little on the warm side. Right now I have seven in a 15 bay 3U Rosewill case. I made sure to space them out for good airflow. When I go to add another vDev I plan to have all the fans upgraded to deal with the heat.
What I can say is that they are very fast, now I need to upgrade to 10GB to make use of the speed...