I decided to just completely rewrite this post instead of trying to keep up with updates.
After about six hours of playing around with Mac OS X Leopard, here are my current impressions:
1. It's not a disaster (which puts it head-and-shoulders above Vista), but it's not a must-have upgrade either. If not for the fact that it contains one particular technical feature that I need for geeky reasons (64-bit Cocoa libraries) I could easily live without it for quite a while.
2. A lot of preferences get lost when you update. In particular, finder view preferences are lost, which I found pretty damn annoying.
3. Spotless plays very badly with Leopard. If you are using Spotless it is important to disable it before upgrading. Spotless ran despite announcing that it was incompatible with 10.5 and would quit, and subsequently spotlight (and hence all search, including mail search) was broken after the upgrade. I don't know for certain that Spotless was to blame, but it seems like a reasonable theory. It appears that I will be able to recover using mdutil to force spotlight to rebuild its indexes, but the jury is still out.
4. I hate the look of the new dock. Even after turning off the hideous 3-D effect (yeah, like that's still going to look cool three years from now) it still looks awful. The black background makes all the application icons disappear into a sea of gloom. It makes me want to slit my wrists every time I look at it. (Maybe Apple wanted to go for the goth market?)
5. Time Machine is cool, but annoying. As far as I can tell, there is no way to turn off hourly backups, so every hour your backup disk starts to spin... If this keeps up I'm pretty sure I'm going to end up disabling Time Machine, despite the fact that it's a pretty spiffy way to do backups.
6. Spaces has a weird bug that manifests itself when you open an application that opens a lot of windows over a significant time span (like Final Cut Pro). If you switch Spaces during the startup process you end up with some of the application's windows in one space, and some in another. The only way I've been able to figure out to recover from this is to quit the application and try again. It's damned annoying.
7. I ran into one really bizarre bug that resulted in movies playing with a sort Max Headroom jerkiness that I have never seen before (and hope to never see again). Restarting Final Cut made the problem go away, but it was pretty ominous looking at the time.
8. I have the general impression that things run faster under Leopard than they did under Tiger, which is no small thing.
On a technical level I have to say I'm pretty impressed with what Apple has done. I know how hard it is to make something like Leopard (it's really, really hard) and the fact that it doesn't seem to have any major problems so far is no mean feat, notwithstanding the odd annoyance. I'll probably get used to the new dock some day too.