I've been running Leopard for just over a week, and it's a really mixed bag. My memory is that Tiger and Panther were only annoying because they failed to fix things I wanted fixed. Leopard has broken quite a bunch of stuff, and I afraid that some of it just isn't going to get fixed.
The whole user interface got an overhaul in the form of the new "unified" window style. I find it nearly totally unobjectionable. The only places it bugs me are where it breaks apps who relied on metal and now have weird looking custom buttons -- and that's the fault of the application authors, not of Apple. The new dock is stupid, but I can turn it off, so that's fine.
It's just all the little things. Stacks are a great idea, but because they're just represented by a bunch of superimposed icons, it's nigh impossible to tell the difference between a stack containing a disk image and a disk image -- and there are many other similar conflicts. I can't usefully right-click an icon in an expanded stack. I can't put a folder in the dock and have it open normally when clicked. I can't get the old popup-menu style navigation for it. Ugh!
The new Finder sidebar seems swell, except that it is totally insane. When I eject volumes, sometimes they go away and sometimes they don't. Sometimes the icon for an external disk is there, though it's been ejected. If I remove it from the sidebar, then the next time I connect that disk, it isn't there. What? It's completely insane, and has at least one led to me yanking the firewire cable of a drive that was neither in my sidebar nor safely ejected.
The once-useful preview pane for an MP3 file used to show a QuickTime-style progress bar. Now it just has a giant, ugly music note. I can click it to play a preview, but I can't see how far into the track I am, and I can't seek.
The new folder icons are wretched.
I used to really, really need to have working virtual desktops to feel productive. When I got sick of all the quirks and bugs in the add-on virts for OS X, I learned to do without. I'm not surprised, but I am sad, to find that I really don't like Spaces. If I'm on a workspace with no Terminal windows and I try to open a new one, I'm moved back to Terminal's home space. I can't do anything to label a workspace, like give it a distinct background or floating text, so I'm reduced to hitting F7 or flipping between spaces to see where I am. It's just too annoying to use.
Time Machine is great. So far, no complaints.
They still haven't fixed the bug that won't let you turn on FileVault if you're on a case-sensitive filesystem.
Terminal is a little better in a few ways. It's more scriptable, and it lets you say that bold text should be in a brighter color. The latter enhancement makes up for the fact that I can no longer use the TerminalColors hack.
The inclusion of a (slighly hidden) Apple Remote Desktop client is a nice bonus. I upgraded my mother's Mac Mini to Leopard and already I've been able to use the "Screen Sharing" app to fix problem several times. Sure, there was Chicken of the VNC before, but Apple's app has a few really nice features. The ability to scale the remote display to fit the local screen is so useful that it makes me wonder why every remote desktop system hasn't had this since long ago.
I'm looking forward to someone running a free CalDAV server so I can take advantage of the improvements in iCal. So far, it just looks uglier to me.
Despite the one totally random Google hit suggesting the opposite, my piece of crap Samsung phone (may it die in a fire) still doesn't iSync with Leopard. I am not surprised.
The whole system seems snappier, too, which is probably the best thing about the upgrade. If the obnoxious Finder sidebar behavior gets fixed, I may be able to forgive almost everything else.
I also sure wouldn't mind some Leopard-compatible releases of things that I'm currently doing without, like WindowShade X. Heck, I wouldn't mind a less twitchy version of my EVDO modem's software (it works, under Leopard, but issues a bunch of stupid warning messages when inserted), and I'm starting to think that I'll want a Leopard version of CandyBar, or some other icon replacement tool, to get rid of these horrible new folder icons.
Here's hoping that 10.5.1 comes soon and fixes things.