We have a Thumblers dual-drum. The smaller sized one. Like with most kids, they were really into it for a while, then kinda lost interest after a few batches. The wife unit runs a batch of broken glass from wine bottles occasionally to make sea glass for her art and jewelry projects of hers.
The constant problem we seemed to always have was 1) shortage of grinding media, and 2) lack of interesting rocks locally to tumble. You can only tumble so much milky quartz, ya know?