I'm no expert, but have used one a few times.
They're really good for making things that normally require long, slow cooking - beef stew, pot roast, beans, stock. They taste just like you've simmered them all day, but it only takes a fraction of the time.
They're also great for canning, if you do any of that.
Using one does take some getting used to, but it's pretty straightforward once you learn the process. I don't have one now, mainly because I don't really have space for another appliance, but if I get my dream kitchen renovation it'll be on the list

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