I got an i3 NUC a few weeks ago. I've had no problems with it overheating, but the fan is very buzzy. No problems with the wifi not working due to heat either, and I'm using an Intel Ultimate-N 6300 beneath a 64GB Crucial M4 SSD. It performs excellently, I use it as a HTPC running Windows 7. HD4000 video is perfect, I've witnessed no issues at all with it.
Reviews have said the NUC is quiet, one reviewer saying he needed to put his ear next to the machine to hear it. That reviewer obviously lives next to an airport, or needs his ears cleaned out.
I've ordered one of the passive cases off Tranquil. I'll of course post up my thoughts when it arrives (hopefully next week), and will get some pics of install.