I find it to be quite bad, the second least comfortable way to play the switch (only option worse is a floating JoyCon in each hand). Pro controller is infinitely more comfortable, and if I'm playing in TV mode I can't think of any reason to use the grip over a pro controller. Kinda funny too since I think handheld mode is perfectly comfortable, but for some reason the spacing when placed in the grip doesn't work for me at all.
I've only really used the grip when playing Mario Kart with my siblings, since we rotate who can use the 2 pro controllers and who has to use the JoyCon grip, and I definitely play worse with it. Holding a JoyCon in each hand really hurts my carpal tunnel wrist so that isn't a better option either. I kind of wish I could understand how so many people find one in each hand so comfortable, since many people seem to swear by it, but it is really awful for me in many different respects.