I had one for quite a while. I had mine outside. The upkeep is similar to that of pools. The PH has to be tested, and they require chemicals to remain safe to use. They also use a lot of electricity, especially if you want it nice and hot. If you use the blower to make all the bubbles, the hot water turns cool within several minutes due to the air being forced into the water to make the bubbles. I was excited when I got mine, and used it faithfully for a couple of months, but then I started using it less and less. I guess the thrill wore off. But even if you don't use it often, you still have to add the chemicals anyways. I ended up selling mine.
Maybe you are one of the people that would use it consistently, and it may be a good investment, but from personal experience, and the experiences of some others I know, it ended up being a waste of money...