A few notes after upgrading to MacOS X 10.5:

• I installed 10.5 via a standard upgrade, and pretty much everything just worked — all my apps were fine, and one of the only things I had to re-install was a custom certificate in my Java keychain (certainly not a mainstream thing that most others would experience).
• Like most others, I abhor the new way that the Dock shows folders. I’m not sure which of the handful of changes is worse, though; between the idiotic popup fans and the way that a folder is displayed as a stack of the icons of its contents, the whole implementation is broken.
• While the new Terminal app is way better than its Tiger predecessor, it still doesn’t know how to handle fonts. For example, my favorite monospaced programming font, DejaVu Sans Mono, looks nigh unusable in Terminal. And without the ability to set keyboard shortcuts which launch connections to remote servers, iTerm still beats Terminal’s pants off.
• The other major thing I had to reinstall was my Cisco VPN client — but the version I was running under 10.4 still works just fine (as does the fab Shimo app which makes the VPN client usable).
• I’m not wild about the way that Finder windows now look just like iTunes, and it makes me realize how little I care for the new iTunes interface (and thus, the new Finder interface).
• Spaces is also pretty broken (like the inability to wrap around to the beginning of a row or column), so I can’t see it getting much use on my computer. That being said, if they fix it, it’ll probably be an amazing tool for someone like me who has a few distinct workspaces open at any point in time (e.g., my programming apps, my clinical apps, and my general communications apps).
• Screen Sharing is plainly awesome — it finally gives users the same capabilities that Microsoft’s Remote Desktop has given to Windows users for years. (And one unexpected surprise I discovered: bringing up the Finder’s “Connect to Server” window and typing in a “vnc://hostname/” URL does the right thing, launching Screen Sharing and starting up a remote session.) The client app does nice dynamic resizing of the remote desktop, and seems to deal nicely with the typical lagginess of VNC sessions… whatever Apple did, me likey likey.
• The new version of Mail.app doesn’t like MsgFiler, my favorite app for quickly filing messages, and it was disabled the first time Mail.app launched. Fortunately, Adam already has instructions online for getting it back working, and the instructions work perfectly (as does MsgFiler).
• I haven’t used it enough to be sure, but the wireless subsystem seems to pick up networks that are in your “Preferred Networks” list much faster than Tiger did; there were times with Tiger when I’d wait 30-60 seconds for my MacBook Pro to recognize that I was in the presence of one of my known networks, but I feel like I haven’t had to wait more than 10 seconds or so with Leopard.
• While I’m on networking, the new interface for controlling network configurations is just a billion times better than Tiger’s. I can’t put my finger on the exact reasons why, but after using it a few times, it feels like Apple finally got it right.
• Finally, while not Apple’s issue, I’m out on a limb on Leopard since my favorite backup app, SuperDuper, isn’t yet compatible with the new OS version. On my Mac Pros, this isn’t a huge issue since one of them is centrally backed up and the other has a big drive dedicated to Time Machine; on my MacBook Pro, I’m anxiously awaiting the new version of SuperDuper.

That’s all I have for now; this morning, I installed Leopard on the third of the three Macs I use in my regular, day-to-day work, so I’m now immersed enough that I should have a few more observations over the coming days to weeks.