Today was my first day of testing with "Leopard", having installed it late in the day yesterday.  I'm very happy to report that I did NOT encounter the BSOD that so many other people reported!  Here's a few quick notes:

• X11 is FIXED!  FINALLY, I can ssh from a Mac into other boxes running X and not have to worry about whether graphical apps will display properly!  In 10.0 all the way through 10.4.x, X over SSH was problematic, sometimes completely scrambling the remote sessions.
• TimeMachine is really cool.  I think...  I'll let you know if it ever finishes synching.  My machine is lightly loaded at best, pretty much just used as a terminal into Solaris and Windows servers, but TimeMachine ran the entire workday and did not finish with that task.  The progress bar stopped moving sometime about lunchtime, but both drives are still grinding like crazy...  First 15GB took about 2 hours, and the drive only had 29GB on it so I'm really not sure what it's doing.
• Leopard did a good job upgrading my user preferences/settings.  Just 2 things that didn't go right...
  • It turned bluetooth on without warning.  Just an annoyance, but since I don't have any bluetooth stuff, I prefer to have it off.  Note that it DID leave my wireless networking off, just the way I like it.
  • It ate my printers!  I had several networked printers (that are a pain to setup) installed.  Am NOT happy about this, but if it's the only major thing that's going to go wrong...
• The new menu bar is hard to read.
• "Stacks" are just plain weird.
• "Spaces" looks to be real cool, basically the same as Unix workspaces.  However, I never found a real practical way to use workspaces (and eventually gave up on them), so I doubt Spaces will be of any use either.

All in all, I'd say that it's a very worthwhile upgrade.  If you're on a Mac that supports it, don't sit around, go out and buy it!  :)