We have one. Had it a year now. Absolutely love it and don't really care if others look down on me for having one.
We use it all year around, we turn it down during the winter so we only use it on weekends, days off etc ... use it a lot more in the summer, most days when it's not raining (I'm in wales so it rains a lot).
It's open, so we can look at the stars and birds etc, we watched shooting stars in it one night, was lovely.
We took the top layer soil off and levelled it roughly and then purchased via the company a levelling deck. The company we used also did the electrics for free and free delivery which included use of a Crane (which we didn't need).
We have had friends over to use it, mostly it's just us and the kids. My husband has some "boys nights" where they go in the hot tub and then kill zombies on the Playstation 😂...like others mentioned it means we talk more, Which is nice.
Fairly easy to keep clean, products can cost depending what you decide to use. We empty it every 3 months ish ... we use a product that is supposed to be better for the hot tub and reduce the amount of chlorine needed. But it needs checked often, we usually check after or before we go in and adjust as needed.
Ours fits 6, could squeeze in more but then it feels a bit too close for comfort. Kids love it too. We turn the temp down in the summer at least during the at anyway.
We used a company called style spas, we paid £6000 all in.
As for other costs, we are also on a water meter, not sure how much it costs extra though as we bought the hot tub pretty much when we bought the house. It adds a lot to our electric bill but like I say we now turn it down when not in use so that saves a bit. It does cost way more than I thought it would to run, but I still love it.