I have not much idea how this Snow Leopard will perform. Although at $18 to upgrade, it is certainly not a bad price at all.
The lady at the sales centre told me that this new OS occupies something like only 9Gb (instead of the usual 16Gb of Leopard OS) and runs faster.
I am usually not very sure about new OS updates, in case after launching there is some bugs.
Will be keeping my eyes on more comments in this thread.
Mostly, there are performance issues, but the new-for-Leopard firewall does not remember settings and asks every time to confirm multiple applications, even though they have been in the firewall list for months. Of course, these could be partly the fault of the developers, as Apple's change caused many applications to fail.
Virtual memory has been a continuing problem since Mac OS X started but with Leopard it made performance worse than 10.4.10. It takes a very long time for applications to quit (10.5.0 - 10.5.7) and with 10.5.8, it also takes a very long time to launch some now.
Some applications don't seem to quit at all with 10.5.8 and Finder is unresponsive, so I have start an application from the Dock, usually Activity Monitor, and things go back to normal.
I also have a problem where all my application menus are displayed at once on the menu bar. This is less consistent so I can't find a way to prove it, other than a screen shot.
You get bug fixes through Apple's software update.
To give you some perspective on optimization, Vista was and is bloatware. Lots of crap that drain the system resources. Windows 7 is an optimized release of Vista, and I'm sure M$ will charge you a pretty penny for it when it comes out. Definitely going to be WAYYYY more than the paltry S$48 that Apple's charging for Snow Leopard.
So you're basically paying for an optimized release of Leopard. At this day and age of obscene amount of bloatware, I'm just a sucker for optimized code