Yeah. I couldn't wait. Just HAD to update. As you can imagine, I do not run your average Mac system, but when I saw that VMWare Fusion, Macports, and a number of essential system level utilities were all 10.6 ready, I took the plunge. It mostly went ok.
First problem: Random spinning beachball freezes. A lot of them. But after seaching around and finding that this was not the general experience with Snow Leopard, I assumed it was something in my system. Since the entire system was freezing, I assumed this was something kernel-level, and indeed it was. I was using Multiclutch, a great little utility that allows you to bind multi-touch gestures to keys on an app-by-app basis. It's for the newer Unibody macs with a touchpad that supports multi-touch. I had it installed, ironically, to disable the zoom gesture in NeoOffice, because it was super sensitive and driving me nuts. Thankfully, Snow Leopard now includes the ability to turn specific gestures off. Not app-by-app, but good enough for now. I deleted the Multiclutch extensions, and the freezes went away. However, to be fair, I also had an ext2fs filesystem driver installed. I disabled that as well, just in case, so it's possible this was also part of the problem (but I doubt it).
Next problem: Firefox weirdness. This is apparently a known issue. Firefox works fine (using it to type this). Just don't open the Help menu. Seriously. It crashes Firefox. But that's ok. I never read Help anyway. That's what Google is for.
Macports was dead after the upgrade. Had to rebuild that, but that is to be expected. No big deal. Just took a couple hours to recompile everything. This isn't a bad thing anyway, because now Macports is all 64-bit.
Fink is another story. It supports upgrading from 10.5 to 10.6, but not without doing a lot of rebuilding. In particular, anything touching the X11 libraries was borked and would not compile. Actually deleted *.la in /sw/lib, which means that a whole lot of stuff has to be recompiled. So, all thinks considered, it's probably better to rebuilt fink from scratch anyway. Fink is not ready for 64-bit yet. Some packages are there. Others aren't. Separate package trees. So sticking with 32-bit for now.
Next, my desktop would not remember its icon positions after Finder was re-launched (via reboot, log-out, or whatever). I discovered that the .DS_Store file was getting deleted everywhere on my drive. The culprit was BlueHarvest - a neat little utility which runs in the background and prevents all those annoying dot files and resource forks from getting created on Thumb drives or Windows network shares. For some reason, after it defaulted to deleting .DS_Store files from my Mac's main hard drive. Just changed a checkbox and that problem was solved.
Boot Camp. Outstanding. The native HFS+ driver lets my Mac's HD show up in Windows as a native read-only drive. I did have an issue with Bluetooth at first. I had to uninstall the Apple bluetooth driver and enabler packages, and reinstall them from the Snow Leopard disk. It was fine after that. Overall, the newer drivers seem more robust.
Finder: Multithreading. One of my major Finder annoyances is gone. In Leopard, Finder displays thumbnails which show the contents of a file. That's nice. The problem is, it takes time to create those thumbnails. Now, let's say you go into a directory of PDF's, and want to rename a PDF. You click on the PDF, hit Enter, and start typing, only to be cutoff every time a PDF icon in that folder is updated. Yes, that's right. You cannot rename a file, until Finder finishes updating every icon. Total BS. My solution was to drop to Terminal, and do a mv from the command line. Really dumb. SL solves this. Now the updates take place in the background. You see them taking place, but any work you are doing in that folder is unaffected. About time.
VMWare Fusion. Yes, this works, but it doesn't work as well. Now, for whatever reason, it looses track of the mouse, and you cannot do anything, whether you are in full-screen view or in a window. You can fix this by switching to a different view and back again. Annoying, but not a showstopper. I assume that VMWare will release an update soon enough.
All in all, far fewer issues than any previous upgrade I have installed.