I'll be installing one in the next few weeks. Dug the trenches and pit for it this past Saturday.
I live in an RV with a 40-gallon black-water tank, and except in the coldest part of the winter the contents liquefy in no time flat. I anticipate no problem with capacity, since I live alone. As for collapsing; I worried about that when researching the method. But you fill the barrels with water before you bury them, so they're always full which should deal with that potential problem nicely. When solid waste enters the first barrel, it displaces the water downstream.
Seriously, it should work great. Plus there are plenty of them out there, and I can't find any complaints.