Well, I received my Leopard on Friday and ran the standard Upgrade installer. Everything went off without a hitch, it's almost as though I've been running Leopard this whole time. Spaces, the unified interface, and Time Machine are all great. The only problem I've experienced is an issue with the BOINC client, which I use to do SETI@Home, but that's not a big deal at all.
Spaces is amazing, I currently have six of them, one for file management and music, the next for chatting, the next for web browsing, the next for eMail, the next for "productivity" and the next for "creative" type apps. Although, I may rearrange all of those again at some point, to put web/email back with chat, and break down creativity a bit, or return to only four spaces. For now, I've got bigger chat and Internet windows than I've had before, and I don't feel the need to
I am very happy about the unification of the interface too; it is something I was complaining about for a very long time. Yeah, a few people's visual cues are gone, but as somebody who uses Windows as well as Mac OS X on a regular basis, my visual cues (and indeed, keyboard shortcut usage) are already out the window.
Time Machine is my new main backup scheme; I went ahead and formatted my 200gigabyte "backup" volume for use as a Time Machine volume. I may eventually get another drive to do the superduper backups to, on a weekly or monthly basis, but for now the time machine has got me covered well enough that I'm comfortable. (Now all I need is the raid1 box for my Aperture library…)
Now we just need to see how this thing holds up in heavy use. It looks like it was optimized for leaving big/heavy apps open on a regular basis, especially since the "new, brighter app-is-running indicator" is so difficult to see under normal conditions. Plus with Spaces, not everything is necessarily immediately visible. Earlier today I had Aperture running for several hours when it didn't need to be, because it was in another space, and the app running indicator didn't stand out very well.
Overall, a great new release, they've tidied up a lot of things, and there are significantly more things in this release that make me happy than unhappy, which is pretty good.