Well first off, you will notice a speed improvements almost everywhere as well as lots of saved hard drive space. After installing I gained around 18GB of space, yes, AFTER installing Snow Leopard from Leopard.
There are a bunch of “apps” that do not work for me. Most of this is because of InputManagers have been removed by Apple. It will be interesting to see what will happen from this and how, if ever, this applications will work again.
So because of InputManagers being removed, one popular app that no longer will work is PlugSuit aside with aFloat. This allowed me to keep certain applications on top of others no matter what. While I barely use this anymore like I use to for my own reasons. I will be able to live without this I am sure.
iStatMenus also does not work and no update has been released yet even after all this time they had to update it. They are working on an update (2.0) and it is in beta already and should be released in the next few days the developer says.
Chax is another InputManager application that does not work. But do not fear as it is its own app now with iChat inside of it. Chax Alpha 1 is out that gives you bare minimum of the Chax features. A lot that you will miss and have to wait for as the developer has time to code and release updates. [UPDATE] Alpha 2 is out and released more missing features that you may of used that I used such as auto-accepting features and bug fixes to some features that did not work in Alpha 1.
Okay, aside from those applications. I have not really had much of a problem with others or have had a lot of time to check all of what I have. Moving on though, I am absolutely still loving Snow Leopard. Expose got a HUGE upgrade that is 10x better than before. Expose now includes minimized apps and even a better interface than before. You will notice some menus like long clicking dock icons look more iPhone-like too.
Overall, I am impressed with Snow Leopard even if some applications need to be updated or may never work again because of InputManagers being removed. Maybe we will see something different with this in the future from Apple or not. I do recommend the $30 or less upgrade. If you are upgrading from 10.4, you can upgrade to 10.6 from 10.4 so do not cave in and pay the even more increased price and save money by getting the $30 version as they are both the same.