For the longest time, I held off on buying products like this one after reading several so-so reviews, or the one or two negative reviews in a bunch of good ones. It took me a long time to figure out that the very existence of an opinion forum will invite some people to nitpick, and often to even find problems where none actually exist. Such is exactly the case with the Klean Kanteen Sport Cap 2.0. I own three such caps, and exchange them among three Klean Kanteen bottles. I've never had the least bit of trouble tightening or untightening any of the caps on any of the bottles. Before writing this review, I went to a local store that sells these, and did another check there, exchanging several Sport Caps 2.0 with several Klean Kanteen water bottles. The result: Not a single problem. The caps all fit well, and tightened and untightened easily on every Klean Kanteen of every size that I tried them on. It's always possible that someone got a defective bottle or cap, but I find it difficult indeed to imagine that the average person would have any trouble whatsoever in this department.
As for flow and noise: The Sport Cap 2.0 is designed to increase the flow, and it certainly does. After a long run, I can drain a 27-ounce KK in well under 10 seconds with this cap on it. It's a cinch to wash it, and other than that it requires no maintenance. The only "drawback", if you want to call it that, is the slight whistling sound the cap makes when you drink from it--not some mysterious, strange sound that will make people wonder, but the ordinary sound of air rushing through a small hole in the cap, past a small silicone valve. The noise could probably be eliminated altogether if the valve hole was bigger--but this would make leaks much more likely (as it is, I can shake, drop, run with, and roll my KKs across the floor with no leakage whatsoever from this cap). In any case, I've used my KKs with this cap at work, in the classroom, and even in the quiet zone of a research library and never gotten so much as a curious glance due to this "noise" that is about as audible as a grumbling stomach.
Unless you spend most of your time inside a completely silent echo chamber, the air-flow sound won't be a problem for you, and as my three-year-old nephew can tighten one of these caps just fine, I doubt you'll find that to be an issue either. If you're looking for an excellent, easy-to-use-and-clean, high-flow cap for a KK, this is just the thing.