The easy answer for 25 bucks for an upgrade, sure its worth it.
What's your hardware and current OS? If everything is working fine on your current system and have no burning desire to move to 10.6, why not stick with what you have.
SL brings some different technologies to the table, like a cocoa based finder instead of carbon, that provides some different polished features. It also has a 64bit kernel if your computer is compatible. It has some technologies for multi-core machines, and other under the hood type changes that really doesn't affect the user experience, i.e., you won't notice it.
That means there's less wiz-bang features for the consumer to get excited about. Again for the price, its tough to pass by, but if you're happy with what you currently have, then why not stick with that.