This product would be 5 stars if I didn't think that it needs to have an ice cream tub, or some other pan in my toilet, McGiver-ing the thing so it works right. I researched how best to potty train my cats, and decided to try the litter kwitter, as it seemed the most sturdy. I, like other reviewers, felt that the step where the hole is first introduced was too big a step. Here are my tips that I think really help you to avoid any accidents, by creating steps that they don't tell you about that have worked for me.
After every time I made a change, either moving their cat box, or changing a ring, I waited a week at least before moving on to a new step. I originally started out by moving their cat box right up next to the toilet. I then put some phone books under it, so they had to step up to get in it. I then put their cat box on an ice chest with stuff inside to weight it down (in case they jumped into it) that made the entrance the same height as the toilet seat, so my cats would actually jump up on the seat to walk into their box (with the lid either up or down, they didn't seem to care). I then used the litter kwitter on the toilet, but did not initially take away their box, because I wanted them to learn that there were TWO places where they could do their business. These starting steps REALLY helped me, as I have a kitten whose brain hasn't quite caught up with the rest of her yet, and she isn't the fastest learner.
I was worried that she would fall through the hole once I changed to the amber ring, because she was so used to the toilet being a solid surface that she could stand on. I decided to improvise, and came up with a solution. I measured my toilet rim's interior diameter and bought a roasting pan (the aluminum ones you buy from your local grocery store.) I then drew a bunch of holes on the bottom of the pan. I decided that when I switched to the amber ring that I would place this underneath, so that if she fell in, she would not fall on the water. I also added some litter, so if she felt down there with her paw, she would realize that she could go to the bathroom in the hole, and not just on the rim, as she kept trying to do. There is nothing funnier than watching your cat play twister on your toilet trying desperately to make their poop land on that tiny ring with the litter on it.
The pan worked really well, but I realized that I could have done this from day 1, and not had to buy this system at all. I ended up just getting rid of the litter kwitter, and slowly cutting holes in the pan, starting with a very tiny hole at first. I slowly cut bigger and bigger holes, until the bottom was gone, and then I removed it completely. I really liked the pan idea, because I could have the toilet seat down. I figured that if they were going to be going with the seat down anyways, they might as well get used to balancing on it. The big con is that my older cat is a big 15 lb male cat, and when he would stand in the pan, and not on the rim, the pan would sink down, so it was more like a bowl than a pan. This was fine, as the pan's lip was big enough that as the pan warped from being bent, it did not fall in. I did use the green ring when this started happening, hoping it would help, but I ended up taking it away again because he likes to stand with his back paws INSIDE the toilet (not in the water, but still inside), even to this day. It also scratched the porcelain on the inside of my toilet rim, but I'm not too worried about it, as it's only a tiny scratch. I didn't have any problems with kitties going on the carpet, but I also took it REALLY slow.
The other plus about using the trays is that they are SUPER cheap, so you can buy two or three, just in case you need to go back a step, but have already cut the holes in it. I would recommend either going the cheap but flimsy way of using the roasting pan, or buying this, which is more expensive, but more sturdy, and adding something as an intermediary step. I have a friend who used a large plastic bowl (like a mixing bowl, or a popcorn bowl) which she filled with water, instead of cutting holes in it, which would be sturdier and wouldn't scratch your toilet. She slowly decreased the amount of litter in the pan (this teaches them not to dig in their litter, or they will get poop on their paws) and slowly replaced the litter with water. This helps if your cat is freaked out by the water in the toilet. That way they will learn not to stand in the hole, or they will get wet, and they will get used to hanging their butt over water. You then increase the amount of water in the bowl/pan, until the bowl/pan is full, then remove the bowl/pan. She trained her cat fast, because by the time she was done, he was already used to standing on the rim over a large bowl of water.
***Oh, and if you have a cat who has urinary or G.I. tract issues, you DO NOT want to toilet train it, as you probably need to monitor how often they relieve themselves.
P.S. I kept the empty litter box next to the toilet the whole time. My younger cat pooped in there once, but stopped when she realized I wasn't going to clean the box. I think this kept her from finding a nice courner of my apartment to go to the bathroom in.
Update: I bought a new handle for my toilet that is an automatic flusher. This senses the cats and flushes a few seconds after the cats move away from it, which worked great...for about a day! My younger cat apparently loves to watch the water flush down the toilet, so she would jump up and down between the floor and toilet in order to activate the motion sensor in the handle, making it flush repeatedly, and I had to take it out. Recently my older cat was diagnosed with a condition that makes him get crystals in his urine unless he's on a special (and expensive) medicated diet. As a result I had to bring out the old litter box again and keep the lid down on the toilet so he would start going in the box again. He now almost never goes in the toilet, but he will if I don't keep the litter box super clean. My younger kitten switches between the two, depending on her mood, so I guess that shows that cats really don't mind using the toilet.