Very good, in fact it may trick you into thinking its bad (smelly).
See it solves a lot of dirt under the toilet rim, and this dirt begins to smell, you might think if goes a way when you flush, but it doesn't.
So I actually thought it was a real crappy product, and wrote that in this product review.
When I've used up 3/4 of the bottle I discover that even if I flush and brush three times, a stream of blue liquid (the product is blue) comes running down the front of the rim, so I use the shower hose and really hot water to flush the under the rim, and that is how I see really brown dirt coming loose.
After this the smell has gone away.