I have only used Capstar when using a spot on treatment like Advantage, Revolution etc, or a spray like Frontline as they can take a day or two to show full effects and I can't stand the thought of ongoing itchies in the meantime!
My cat came from a country pound last year and had Revolution and Capstar together due to a massive infestation. He went mad for about 20 minutes - the fleas must have gone into overdrive! - and then slept like a baby for about 10 hours. I shudder to think how hard it must be to get proper rest with crawling and biting fleas!! He only needed the Revolution after that. Capstar only gives that initial knockdown.