This replaced my stock cooler on i7 4790, a non K version, in a mini ITX case- Corsair 250D, it's not the smallest mini itx, but still, air flow is much more limited than in big cases.
without any other fan in the case, idle temperatures were close to 40'C and under 100% all core-thread load they haven't exceeded 68-70'C, can't say it's a massive improvement over stock cooler which did similarly on idle and only slightly worse under load- around 75-78'C max.
Best thing about this cooler is it's compatibility with virtually any motherboard- it's compact and you don't have to worry about covering PICe or RAM slots, it's difficult to find a right cooler for mITX board, so in that part, this really fits almost everything.
Noise levels- very low, can't say it's silent, well, it can't be silent, especially under load, but if you have any other fans spinning in your case, they will be, most likely, louder than this is. I have 3 other Noctua fans in my build and under 100% load, noise levels were just under 30db, haven't made a super scientific research though, more like consumer type.
Would definitely recommend this for any mITX or HTPC builds, running smaller TDP processors. Noctua don't recommend this for K series, but I'm sure with lower spec i5 or i3 K CPUs you can get away with this. 6700K or 4790K- no, those things gets pretty hot when pushed hard.
So certainly it is operating very well indeed for my use case, while producing a noticable reduction in the audible whirring in comparison to the standard stock fan. The construction looks and feels solid, giving off the sense of premium engineering much like an Apple component. Installation was straightforward and packaging was immaculate. I cannot fault it in anyway, hence, I am one pleased customer.
Cool, quiet, tiny. Using it to cool a G3440 in an In Win Chopin, (mini-itx case, 3.3L internal volume - tiny) as far as I know, this is the only cooler that will fit - hardly a downside because this thing is excellent. It's so quiet that when I first turned on the system, I genuinely thought the fan wasn't spinning. Mounting system is simple, non fiddly and the supplied thermal paste is high quality. The metal, 3M addhesive backed, Noctua badge is a nice addition.
First Time Noctua buyer. Was really impressed from first opening the box. Wasn't anticipating everything being so nicely packaged, clearly a quality product. Took 5 minutes to install and has made a world of difference over the stock i5 cooler that I had. Silent and more efficient. Very impressed all round. Am now a Noctua convert.