I don't know about the spray, but I can sure vouch for the Feliway Comfort Zone diffusers. With six cats in the house, the diffuser is the only way we can keep the peace!
It's not perfect, of course -- multiple cats will always have their spats. But with the diffusers in place, they are far less frequent and far less intense. We can tell the difference immediately when our diffuser runs out!
So I highly recommend Comfort Zone -- BUT DON'T BUY IT IN A STORE. Our Petco charges $45 for the diffuser itself (which comes filled) and $28 for a single refill. Someone here on the site recommended a Kansas company called valleyvet.com, and we should be receiving our first order tomorrow: a diffuser with fill, plus three refills, all for $68. Free shipping, too!