I can't smell it but it seems to calm the cat!
I recently brought home my 5 year-old British Shorthair, a retired breeding queen, who had been fostered while waiting for me to collect her. While with the fosterer, she had nervously refused to leave her travel basket for the first 2 days! Fearing the worst, I purchased the Feliway diffuser and plugged it in one week before she arrived. I also sprayed the inside of her new travel basket with Feliway spray just before I picked her up. When I arrived at the fosterer's house, the cat calmly got into the basket and - give or take the odd meow - travelled happily in the car for the next two and a half hours. When we got home she stepped out of the basket immediately and proceeded to wander around the flat as if she had always been there. That first evening this "nervous" cat jumped onto my lap and has been affectionate and calm ever since. I can only put this down to the Feliway, although I couldn't (and still can't) detect any scent from it, apart from the initial smell of alcohol when it is first sprayed or plugged in (which cats hate). On the basis of this experience, I would recommend Feliway for nervous cats, for use in a pet carrier or to help cats that are about to be introduced to an unfamiliar environment.