I finally decided to bite the bullet and buy the Bubble Unit, my cats are 14.5 and 13lb i was a little worried that they wont fit. I set it up and took a cold turkey approach. I barely had time to set it up and my male cat went in and blessed it :). My female was a little harder, she was somewhat scared of it a bit, but she did use it when she had to go. no accidents.
The attached picture is the morning of day 2. The male cat is using the box and female is waiting for him to finish. when he was done she went in and used it.
i am still using it in manual mode, since its been only 3 days, but so far its awesome. No smell, my cats using it fine. even when i turn it on to do manual cleaning both the cats are very interested in it and not scared of it.