I have. I've also used an outhouse and porta potty. They're all gross, by common standards. Doesn't stop me, but I know plenty who won't use any of the above. Period.
Oh come on Minn. By "common standards" most of what we do to live on a small boat is equally unusual. That's a silly criticism. The commercial composting heads function almost identical to every other toilet. Sit, open a trap door, let 'er rip. What's so different? What's gross?
If you're depositing in it every day, the moment you empty it you have nearly uncomposted waste in the mix. What would you do with it aboard to let it compost for 3 months? Therefore, dumping past the limit is really the only option and I doubt their are many that find themselves in legal dumping areas often enough to be practical. Therefore, the same raw sewage from yesterday's deposit goes straight in the trash I suspect.
This is a valid criticism. It's true, the last deposits have not been "processed" in any real way. This is why I agree it should not be put into the landfill stream, but as others have pointed out, the impact is negligible. I'm sure all the dirty diapers have a far greater impact. But I agree. Don't dump it into the garbage.
If you can't go out to sea, or find a wilderness zone to dump, or otherwise get the solids into a sewage treatment, then NH makes it easy to have two bases that can be swapped out. The lid allows for air flow so the composting process continues. Swap it out, let it cook, then dump it into the dumpster after the requisite time.
Better still, stick with your standard marine head. They work fine. I'm not saying everyone should switch to composters. There are significant benefits (for those who are not grossed out by it all), but it's not perfect, and there are some downsides. Just like with most things in our sailing/cruising world, there is no one right answer for everyone.