My cat had been infested for about six months with fleas that seemed to come back no matter what I did, or how often combed her.
I'd tried Program, Frontline and a clinical-strength injection that the vet gave her but nothing really seemed to work, even when overlapped and vigorous flea combing utilised.
However, after seeing the positive reviews this product received I decided to try it.
Within two days my cat was flea-free and loving it, with only one follow-up spray of her bed and general haunts required to ensure she stayed that way.
I will qualify this seemingly unanimous praise by saying that this is not a complete solution to a flea infestation. I would recommend this, some Frontline as a long-term preventative and some Johnson's 4Fleas pills which seemed to eliminate all the fleas on my cat within an hour or two, all administered on the same day.
Just spray this stuff in the cat's bed, around their litter tray and any areas of carpet or flooring where they walk frequently (NOT near their food!), and you should be golden.
One spray bottle of this was enough for me to thoroughly de-infest a medium-sized house, although my cat doesn't usually go in one or two of the rooms. You may find you require more than one.
TL;DR it's excellent stuff, works almost immediately. Just be thorough and check your cat for signs of re-infestation frequently (especially during summer months).
My cat has been flea-free for almost four months at this point. I know it's said a lot, but I don't know why I didn't try it sooner.