What’s so hard about taking less than one minute a day to manually scoop a couple of clumps out of the box?
I only have one cat, but it doesn’t seem to me like it would be worth the extra expense to buy and maintain one of those contraptions.
Besides, the high point of my cat’s day seems to be watching me humble myself cleaning up his mess. 
Peace,
mangeorge
I must have clicked “new” instead of “reply”. I was replying to this thread;
http://boards.straightdope.com/sdmb/showthread.php?threadid=70122
Sorry, it’s late. And I’m gettin’ old. 
Peace,
mangeorge
I’ll close this one then. You can C&P your reply to the other thread, if you haven’t done so already.