Follow-up review, and it's not good.
I first bought one of these at the start of the year as it sounded good in principle, and it did work - to a point. But even using a very fine litter like Tigerino clumping litter, clumps would still stick to the inside of the main tray area and prove very difficult to dislodge. There was also an issue if any clumps formed in the filter grill as the litter set and blocked the filter from working. The filter insert itself tended to pop out of its bottom housing when the tray was rolled which meant that the tray needed to be taken apart to refit it. On top of this, the plastic started to crack and come apart where it folds over along the top, leaving a large gap that allowed a lot of clean litter to be lost into the removal tray. When I reported the issue with the cracks to zooplus, they kindly replaced the entire tray for me. I had high hopes for the replacement but this too developed the same cracks and I found that it's not possible to buy a replacement filter insert on its own. So I now have two litter trays that are not fit for purpose. My cats love them as the entrance is low and very suitable for one of my cats who has arthritis but I can no longer roll them to clean them so they need to be cleaned manually which defeats their purpose. For the price, I definitely think they're a waste of money and I have given up on the idea of self-cleaning litter trays and am returning to the old fashioned manually cleaned trays which will cause me less stress and save me money on litter.