After reading Matt Woodward's update on his Leopard experience, I thought I'd share a bit of mine.
I really like Spaces (virtual desktop), and I love the screen sharing. In the past I used an OS VNC clone to connect to my desktop from my laptop. The resolution wasn't great and the speed was so so. The new built-in screen sharing performs much better and the overall quality is just incredible. The main feature I haven't had a chance to play with yet is Time Machine, but as I use Mozy, I don't see a big need for it. As others have mentioned - the ability to do tabs in terminal is a small thing, but darn convenient.
I've run into two programs that are flakey/unusable. Cord, an OS RDP clone, has redraw issues. The developer is aware of it though - and in the meantime I've just switched to Microsoft's RDP client.
Connect360, a program that lets your XBox 360 talk to your Mac like it was a Windows box, is dead in the water. I don't see any mention of it on the developer's web site. I've sent in a support request. In the meantime, I'm just going to hook up my IPod to the 360.
My laptop is having issues. Before I installed Leopard, I was having battery issues. The health was a meager 30-ish percent, which gave me like 30 minutes on battery. I've already got one replacement battery from Apple about 5 or so months ago. I guess I need to call them again, but I never got the darn support plan for the laptop, so I may be SOL. That's not the scary issue. Since Leopard, the laptop will reboot. No BSOD mind you - just a sudden and instance reboot. So far it has only happened when idle. I'm not entirely convinced it is Leopard though since the battery issues could possibly be involved as well, although the reboots happen on AC power.