I gave my cats a taste test by comparing "home grown" to store bought (from Wal-mart) and they ignored the store bought entirely when given the option.
BTW, it's easy to grow, harvest and dry. Cut it after the first bloom, leaving 1/3 of the stalk in place until the end of season (it will continue to grow). Tie the stalks together at the base with string and hand upside down in a dark space for a few weeks.
Hanging it upside down causes the active ingredient, nepetalactone, to flow down into the leaves. Additionally, nepetalactone is apparently sensitive to light (loses potency), hence the dark drying room.
Next year I'm going to try to grow a HUGE patch of it.
A word of warning: catnip can be very invasive. It literally grows like a weed. I never minded, and actually like the sweet lemony smell, but I’ve seen it quickly squeeze out other plantings if left unchecked. I’ve heard it it grows very nicely in a container.