Totally fine, no major complaints off the bat with these. The charger isn't the best design- it only tells you if it's charging or charged, without indication of exactly how charged the batteries are.

Time will tell if the actual batteries hold up- it's been about a month and they haven't gone bad, so at least I didn't get any particularly terrible ones. I had another set from a different brand where one battery took 6 months before it became unusable, and the other battery took a year. Meanwhile my original Sony branded battery is 2 years strong and shows no strange behavior, just *maybe* slightly lower capacity than my newer Sony branded batteries. It seems to be a trend that these non OEM batteries just aren't made with high quality cells, so hopefully K&F Concept's batteries will be a change from that. I've liked K&F's other products, so hopefully these won't be an exception.

I keep all my batteries labeled (A, B, C...) including their first use date, and I evenly cycle between all my batteries (after A is done, I move to B, then C, etc... until looping back to A). That way, none of my batteries get used harder and more often than the others. In turn, that allows me to catch a bad battery early, and know that it wasn't because I used it far more than the others. Hopefully I'll remember to update this when these batteries finally go kaput (or at least start acting funny).