I think these improvements are definitely in line with being more productive on a Mac.

You can now do phrase searches in Spotlight in a tip of the Google iceberg style (a complete list of Google search operators can be found here) by putting your search terms between quote marks. You can also do quick calculations with 40 mathematical operators (geeks rejoice).

Preview allows you to annotate a PDF file on the fly and finally select its text. You can also do an alpha channel (transparency) just like in Keynote on images to detach the subject from the background. You can send an image directly to iPhoto.

iChat allows you to open multiple IM accounts at once, it has three levels of compacted views to squeeze more conversations in a given window. It can now record a voice chat and a video chat (great for podcasts).

Finally the Finder is improved with the possibility of displaying the path in the Finder window. I like the look of the new Finder and its ease of use. I especially like he way the Dock stacks up my downloaded files and my Applications. I really enjoy Spaces that enables multiple Desktops (stop laughing Linux fans).

For those who want multiple Desktops (upgrade or not) I recommend Desktop Manager, a neat little utility even more powerful than Spaces. Hope you enjoy these features, I recommend you upgrade to Leopard and do the upgrade to 10.5.1 immediately after. A clean install is the way to go, provided you have backed up your user folder previously.