I just installed Snow Leopard and I must say that I am very impressed. Apple has once again done an excellent job and rounded off the rough edges on Leopard, which was already a pleasure to use.
Although Apple recommends doing an upgrade install over a clean install, I opted for a clean install because I've modified various parts of my OS X Leopard installation over time (installed Python 2.6 over 2.5, installed KDE for OS X, a few MySQL installations/upgrades, etc) and I wanted to try a fresh start of OS X. Formatting the hard disk took and the installation was about half an hour from start to finish, and I was able to use my new system as soon as the Snow Leopard installer rebooted the machine for me.
One of the first things I noticed was that everything was much more snappy compared to Leopard. Everything I do seems more responsive while using Snow Leopard; all of the small lags or delays (while restoring a minimized window, for example) are gone. On the other hand, more intensive tasks like opening a large application or restarting the machine are noticeably faster. Phoronix released an article today comparing the performance of Snow Leopard (OS X 10.6) vs Leopard (OS X 10.5), and at first glance it seems like OpenGL performance has dropped a bit, but otherwise there are enhancements all around.
I was a bit worried as I was upgrading because I had read this document describing that certain (popular) software titles were incompatible and would not open on Snow Leopard. As it turns out, I had no problems at all with any of my software. All of the software I was using with OS X 10.5 works perfectly with OS X 10.6, including Thunderbird 3.0b3, MacFUSE+NTFS-3G, Starcraft and OpenOffice.org. It should be noted that for Starcraft and other PPC-based applications to work, you need to install Rosetta from the Optional Installs of the Snow Leopard installation DVD.
So if you're reading this on a Mac with OS X 10.4 (Tiger) or 10.5 (Leopard), I recommend that you grab a copy of Snow Leopard the next time you are nearby an Apple store or reseller. Snow Leopard is being sold at $35 for a single license or $55 for the family pack (5 licenses for use in one household) and the feature and performance enhancements it brings are well worth it.
For what it's worth, one of the changes I particularly like is how Snow Leopard reports disk size, which as documented here, has changed. All tools in Snow Leopard now report disk size using base 10 measurements, meaning a 200GB hard drive appears in your system as 200GB and not 186.26GB. I'm glad that somebody has finally made a move, because users - myself included - honestly don't care if a GB is 1024 or 1000 MB. That is irrelevant. What does matter is that the measurement doesn't change in one context and another. Otherwise, it isn't much of a standard measurement, right? It would be ridiculous to propose that the same unit "kilometre" represented a different distance depending on if you walked or drove from point A to B, but essentially that is what has happened in the computer industry. Depending on if you are shopping for or writing information to a storage medium, its size changes with the same unit. It's pretty odd to explain that when someone asks "where has the remaining 300MB gone on my 4GB USB key? I only see 3.7GB."