I've used them on and off for the last few years, trying to achieve the laziest level of stability possible with my tank. It's hard to say if they've really done much for me, because high nutrients had my Ca/Alk consumption all out of whack. So when I'd test, I wasn't seeing a whole lot of a difference, I just had chronically high calcium and it kept it that way. That said, I do believe it helped me hold out a little longer between water changes and I definitely didn't see any downsides. They used to be fairly popular, I believe they do work, but my tank wasn't really a good candidate for them. But as mentioned above, there are better methods available today.
I recently switched to All For Reef and I don't think I'll ever look back. I avoided buying a dosing pump for years because I'm cheap, but I'm happy I finally did it. My corals all are looking great, my macros are looking much healthier, and over all my tank is way more stable than it's ever been. After buying a tester bottle of AFR liquid, I bought the jar of power to mix it myself. I'm sure I'll adjust over time, but at my current rate, this powder should last me 2 years and 3 months for $50, which is pretty damn cost effective.