They apparently work fine, but they operate at higher salt levels so be sure that's OK with the manufacturer of your tub. Salt can be corrosive to lower quality stainless steel and other metal components, though the level required is 2000 ppm compared to saltwater chlorine generators in pools that typically use 3000 ppm.
There is inconsistency in whether the product produces 15 or 32 grams per day (different parts of the website say different things for their different products). To put this into perspective, one person-hour of soaking requires around 9 grams of chlorine to oxidize the bather waste. So unless you have multiple people soaking every day, the unit should be able to keep up. However, just as with manual dosing, it is easiest if one is soaking regularly every day or two since one setting then has the chlorine level be fairly consistent. If you instead just soak once or twice a week, then it's trickier even if you use the "boost" mode after each soak. You can probably figure out a setting and routine to make it work well for you -- meaning that you start off your soak at a reasonably low chlorine level of 1-2 ppm so that you don't notice the chlorine much during the soak. They recommend 3-5 ppm FC, but you are very likely to smell and notice the chlorine during your soak if it is at that level. The unit does NOT output chlorine very quickly so there will be a time during and especially after your soak where the chlorine will be used up, but hopefully not long enough for biofilms to form.
One thing they don't talk about in the manuals is initially adding any cyanuric acid or Dichlor to your spa after a fresh refill. If you don't do that, then the active chlorine level will be very high and will react with your skin and swimsuits faster as well as outgas faster (so will smell more and get used up faster and react with the hot tub cover faster). So for better results, I suggest you get some CYA into your spa water after a fresh refill.
