MacDevCenter has Tiger upgrade advice, as does MacFixIt. MDC advocates a backup-wipe-install, though mostly as an excuse to clean up your files.

For the record: L did an upgrade install, and suffered kernel panics and reboots for a bit until it all settled down and now seems to be OK. I did a wipe-and-install, and forgot to de-authorize my Audible account, so now I need to beg Audible to let me re-authorize. Backups are essential; some software I needed I could only obtain from my backups. If you don't have a nice external hard drive yet, buy one and use it before upgrading any operating system on any computer.

Safari feels faster and more robust, my biggest wish for Tiger. The new Dictionary is fabulous (hadn't heard of it before, a nice surprise), and well-integrated into the OS. Hate the new Mail buttons. QuickSilver seems faster than Spotlight for launching apps, but they both do so many different things that I'll probably use each of them. Dashboard is cute, but I keep forgetting it exists; was that way with Expose at first too, I suppose.

Quick note for the Emacs-on-Mac-from-CVS folks in the audience that haven't already figured it out: Emacs needed a bit of a tweak to work in Tiger, but that tweak has been integrated into the latest sources as of a couple of days ago, so the usual checkout-build-and-install instructions will work.