I don't use a Mac but you do not have to purchase the software. It's just an upgrade of the current operating system. Apparently they have trimmed it down (out of the ordinary for a technical company) and it is supposed to run faster and take up less room on your hard drive. It's supposed to be an excellent upgrade (even if he wasn't so cute I would still trust David Pogue
) but if you're happy with what you have there is no reason to spend even $30 upgrading. Pogue (not CP) does mention that there are some programs which don't work as well with Snow Leopard but those kinks should be ironed out as time goes by.
BTW Macolytes, this is not intended to be a Mac bashing post at all. I feel this way about software in general. If you're happy with what you have (Windows or Microsoft) you don't have to buy something new. Our photo editing program is from 1996 and it works beautifully. I stopped using Norton virus software because it became a huge bloated unreliable mess.
It's all marketing, making you think that if you don't have this your life will be less worth living. I don't think so.