I used it this morning in order to stop a check engine light from coming on due to a catalytic converter reading. My initial impression is positive as when I put it in, following the directions, and started my car--a plume of smoke came out after a couple of seconds. The car, a Mercury Mystique V6, seems to run a tad smoother and peppier, but nothing dramatic. The check engine light seems to come on periodically, about every one or two thousand miles between reset. Last night I finally had a tech-friend read the code and he told me my converter was going. I've also noticed a steadily, but slight drop in mileage from about 26 mpg in mixed, but mostly highway, driving to about 23-24 mpg. So I thought I'd try this out of curiosity as much as avoiding paying $400-$1000 to fix the exhaust system--as I plan on dumping the car next spring anyways...
I purchased the Cataclean bottle from a parts distributor for $25 and the stuff is made here locally in Orchard Park (I think this is their U.S. facility, but the product originated in the U.K.). The guy at the counter said he's gotten a lot of great feedback and started making almost 'snake-oil' sounding claims. But I do think it definitely did some cleaning as evidenced by the smoke. We'll see if the light stays off...