Mac OS X resembles the classic ideas of Macintosh Operating system. However, it does not have the same overall feel. OS X is not as "snappy" as Mac OS 9, especially on G3 computers. Nine is much more easily customizable in looks than OS X. I'm sorry, but the aqua interface is not very pleasing to me.
Now for you older users. I would definitely suggest this as an upgrade to OS 8 both functionality and compatibility.
Overall, Mac OS 9 is a great system to have around. Even if you are all-out OS X but have used "classic" operating systems, keep OS 9 around because it sometimes feels good to come home to this wonderful interface.
Finding OS 9 for sale these days is a bit tricky, though, so I was quite pleased to find it here on Amazon -- good luck finding it listed on, say, the Apple Store!
So if you're running, say, an older G3 model on version 8 of the OS, and don't want to (or can't) take the OS X plunge yet, I suggest that you grab a copy of OS 9. It's quite a bit more stable and functional than its predecessors.