I've just finished installing Leopard on my laptop and so far, so good.
I started off doing a simple backup (copying folders to an external drive) and then did a full Erase and Install as I always like to do. Installation itself was a breeze.
After installation, the first thing the OS wants to do is build the Spotlight index. I made the mistake of having the external drive plugged in when this started and it was reporting it was going to take 2 hours to build the index. Fortunately, I realised what was going on and told Spotlight not to index the external drive (which it honoured immediately).
After Spotlight indexing, Leopard asked me if I wanted to use the external drive as the Time Machine drive. I said yes and it proceeded to basically make an entire copy of the initial Leopard install. For every snapshot Time Machine takes, a directory tree is created on the external drive. Files that haven't changed are hardlinks shared by previous versions. It's a pretty neat way of doing it—it basically means you can navigate to any snapshot and it looks like the full filesystem. The disadvantage of the hardlink approach is entire files are stored, not deltas, so if you change one byte in a 100MB file, the 100MB gets stored twice.
My Address Book and bookmarks didn't sync via .Mac as I'd hoped so, for the bookmarks, I found the .plist file in the backup and copied it over. The Address Book was a little more involved as it appears they changed the file format. Fortunately, I hadn't upgraded my MacPro yet so was able to copy from the backup to Tiger on my MacPro and export from there in a format that Leopard could read in.
Oddly enough, though, my Mail accounts did sync via .Mac so when I opened Mail.app, it started the lengthy process of downloading 2GB of mail over IMAP. The left pane is ordered differently (my non-Inbox IMAP folders were underneath my smart folders rather than above as before) which threw me at first but I quickly got used to it (and actually prefer it now).
Overall the UI seems crisper. The translucent menu bar isn't really my cup of tea but everything else I've seen so far seems to be an improvement visually. It's certainly nice having consistency between windows of different applications.
Spaces so far is working just like I had hoped. I was worried for a while before the release that different windows from the same app might not be able to live in different spaces (a disaster for something like Safari if you have different spaces for different projects). But fortunately my fears were unfounded.
It's a small thing but configuration of Terminal is much much nicer now.
It's always fun looking at what applications people first download when they install a fresh OS. In my case some of my usual suspects come out of the box. I didn't need to download Python 2.5. Nor Subversion. That basically meant TextMate was the only thing I had to install to be able to get right into my open source projects. TaskPaper is the only other thing I've installed so far.
There's still a lot I haven't tried yet. I can't comment on what I think of Stacks yet. Or Quick Look. And I haven't had a chance to try out iChat yet (neither the fun stuff like effects and backdrops nor the serious stuff like application sharing).
But overall I'm delighted by the upgrade so far.