Onsen are great (being out in a starry night in a rotenburo (outdoor bath) with a light snowfall is a must-experience IMO), but a lot of what you're paying for will be the traditional ryokan (traditional Japanese inn) experience and kaiseki (traditional multi-course Japanese dinner) that usually accompanies an onsen stay.

Personally, I'm a big fan, and I'd recommend everyone to give it a try if they can (but to also research the manners, rules, and exceptions - tattoos are a big no no in most public onsen). For those visiting an onsen for the first time, you can use a site like https://selected-ryokan.com/ (which includes private onsen) or (my recommendation) Booking.com to find something tourist friendly and read the reviews.