I've had 2 kernel panics (one last night) on a year old MacBook Pro with Leopard. Something about freeing buffers that were already freed. Somebody definitely missed that in quality control. That said, it is a .0 product, no service releases at all, and I've had kernel panics in Vista too (at work). But it's definitely a lot buggier (for me) than Tiger was when it came out. But I like the new features, I have a hard time thinking of features that I want that it doesn't have, Tiger had several areas I thought were lacking (Finder, Mail, printer configuration, network configuration, etc...). I'm confident that Apple will have the issues fixed in some point releases, Tiger at .10 was rock solid.
I had some weird bugs in Leopard Mail.app as well. It got into a loop where it was trying to add mail to the "Sent" folder on an IMAP server, but the IMAP server would return an error, so it would keep trying. But apparently it was actually working while reporting that it wasn't working, so I had the same mail in my sent mail folder 35 times before I finally deleted the settings for the IMAP server and re-added it. It's been fine since then.