First, some advice. When booking, I recommend a creek-side tub, around sunset or a little after. I find the babbling creek to be very nice.
When you arrive, you check in at the front desk, and then are directed to one of the campfire areas where you wait for your tub to be ready. The waiting is okay... there are campfires and benches for you to sit on. The only thing is, the staff is very simplistic and mechanical. They do their job by cleaning the tubs, letting you know when one is ready, and letting you know when to get out, but they don't seem happy to see customers. Don't get me wrong, they are still attentive to guests, but they're not likely to treat you like you might expect to be treated in a spa/resort.
Once you get guided to your private tub though, none of that matters, which is why I'm still giving 4 stars. The tubs are large, kept very clean and the water feels so refreshing and pure that you come out feeling like a new person. An hour is enough, but I gotta say the one time we did two hours, it was twice as nice. I recommend bringing along a bottle of wine to sip on and truly let go.
When I tell others about this place, I find people tend to respond with somewhat nervous questions like... "is it reallllly private?"... or "what if I didn't pack a swimsuit?". I'll address this. Yes, it's really private. You have 3 full walls and a ceiling. The 4th side is open only to the forest. The tubs are far enough apart that you're not going to hear other groups or be heard. As for swimsuits, they're not required, and it's completely up to you and what you're comfortable with. The point is to relax after all. When your time is almost up, the staff will come and knock on the wall to let you know to get ready to come out.
Hot Springs is a little out of the way, but well-worth the trip. I also recommend the restaurant across the street (Iron Horse Station Restaurant).