Ummm... I think you need to do a little research on catnip. It is a herb, it's related to mint, and it's totally safe. Some cats don't even react to it... others - I'd say the majority - go quite funny on it, like you described. Some cats eat it, too, and like I said, it's totally safe.
A cat rolling around is just a cat having fun for a few minutes, it's not something I'd give every day but it's something they enjoy... where's the harm? There aren't any side effects. Catnip started growing in our front yard as a volunteer and our cat can occasionally be found having a little fun in the flowerbed.
I also found it helped relax the semi-feral I'm trying to tame; he'd be going berserk on me at first growling and hissing, but I'd throw him some catnip and he'd settle down, win-win for both of us, as him being that worked up wasn't helping either of us.
And it is often suggested to help cats learn where to scratch, it's a great tool when sprinkled on a scratching post that will guide them to it instead of your furniture.