It was completely painless; just click install and it all just worked. Its great to finally have a real Java 5 now (after using a binary someone gave me from last JavaOne).
Spotlight's cool, I love the new Mac.Mail features (especially multiple machine sync and better multiple account handling), I really like dashboard - especially the water effect when dropping new widgets on the desktop (yeah, I'm a sucker for eye candy :).
I think like Dion my favourite bit of Tiger is the RSS screen saver - really cute!
So far the only downside of upgrading is that the CodeTek virtual desktop no longer works :(