The Bayer Advantage for cats gets mixed reviews on the internet and most of the negative reviews appear to be that the application was too strong or that it is too expensive. Overall it still has the best reviews overall for an anti-flea solution for cats. Bayer Advantage for Cats comes highly recommended by vets and it does work fast. You will see results within 24 hours of application on the back of the cat. Note that the fleas will come back within a week unless you also get an anti-flea remedy for where the cat sleeps. From the reviews, one notices that in New South Wales in Australia many are finding the application not to be as effective on fleas as the oral tablets. This might be a regional issue. Note that you should always consult a vet when you have a flea problem. Many cats are allergic to flea bites and it causes allergic reactions as well.
Many users of the product have noticed that within 2 hours of application the fleas start falling off but that the cat has an itching problem which leaves the cat scratching for the rest of the day. As stated you need to consult a vet as an allergic reaction to flea bites might not also be seen. Some have also reported that the cat became lethargic for a day or two and one user had taken his cat to the vet after application, however, there was no toxicity discovered from a blood test so the product is very safe.
Overall a very good product with only a few who have cats that showed a negative reaction to the application of the liquid. Note that you need to buy the correct product where the weight of the cat is taken into account. As an example, this application is only for cats over 4Kg and not under this weight.