It's pretty awesome. I know they had earlier overheating issues, but were able to solve it with a thermal pad in newer models and a BIOS update. There's also alternative cases you can buy like
http://www.silverstonetek.com/product.php?pid=437 which are much more efficient at cooling the NUC. It's got Windows 8 and an SSD, so it boots up in like 7-8 seconds, and it's powerful enough to run anything I've thrown at it thus far. I got the Celeron model, so it probably can't run 4k or anything, but anything else is gravy. I'm sure the i3 or i5's would fare better if you needed 4k for whatever reason.
It's been a godsend though. Was absolutely getting destroyed by electricity costs. I had two of these work statations (
http://h18000.www1.hp.com/products/quickspecs/12849_na/12849_na.pdf) that were given to me by my dad. One was being used as a PC/HTPC and the other a server, and the things cost about $20-40
each per month to run depending on how much I ran them - which was often. I've switched to an Intel NUC as an HTPC and a Raspberry Pi w/ external HD's attached as a small footprint NAS and the difference is pretty damned noteworthy.