I bought one like this from Bass Pro on clearance (maybe $5) and it works fine. It's more of an "emergency" use thing though.
The bulk of the time we use cat holes, but this came in handy for when it is raining or in the middle of the night, and not really convenient to go for a hike. Much nicer than getting soaked or having to get dressed and put on boots. I could also see using this in places without much privacy.
A caution for those that mentioned using grocery bags; not sure about your local market, but the bags ours use are not very water tight and often have small holes. I would really hesitate to use them for human waste or you may be stuck cleaning up some nastiness. If you are forced, I would recommend at least doubling up, if not triple. Heck, even some of the thinner garbage bags aren't very water tight.
Fortunately, I have only used the bags that came with mine so far (only actually used it in the tent like 3 times), so haven't had to look for other sources.
BTW, I like this one because you can also use it when using a cat hole (no bag). It's much nicer to sit than squat. In my Army days I would just one cheek it on the folded blade of my E-tool, but after getting a little older and heavier, that gets a little old. You do have to be careful with how wide of a hole you dig. I do more of a slit trench that runs "north / south" between the legs. Also need to make sure the ground isn't too soft as these legs will sink in if you're not careful.
I like the kitty litter idea, mainly for cutting down the smell and for soaking up the urine. I will probably throw a handful in each bag and roll them back up before my next trip so they are pre-loaded.