These work great. Nintendo should have included these in the original package.
But given that they only do 2 things, so value for money is not so great. $15-$20 would have been better.
They offer a grip which is not all that great to begin with. The pro controller is a much better option or 3rd party options that offer better ergonomics & an overall better feel are a better option. Otherwise, using the joycons on their own is more ideal.
They offer a charging ability which is really all I needed but I didn't want to trust a 3rd party device to do that for me. That's all I use this for, is to charge my joycons since the Switch doesn't charge them while it's off for whatever reason, despite being able to charge itself.
It's pretty easy to use, no instructions needed if you already know how to use the Switch, the joycons slot in the same way. It comes with a charging cable; surprisingly they don't charge extra for that.
It's the real thing, not some knock off and does what you'd expect it to.