I just upgraded to Tiger (Mac OSX 10.4). The guys at work gave me a new image after I had backed up all my data, then I (slowly) pushed my data back onto the machine.
I found I was wasting a lot of space, I should really clean some stuff up.
As for Tiger, no real noticable improvements yet. Spotlight is neat, of course. So is Dashboard.
But what about Automator? There has been so much buzz about Spotlight and Dashboard, but Automator was supposed to be on par with those guys. What a slacker.
I'm guessing we'll see a surgance of neat Automator utilities in a while, once everybody settles down and finds a use for it. I love the idea of scripting applications in a sharable manner.