There are also a variety of subtle enhancements to program operation, including the ability to apply graphic styles additively. A few changes to the panels include a modification of the Appearance panel's operation--it's now an attribute editor, allowing you to toggle visibility or adjust parameters like opacity, blend mode, and fill color--and the addition of a basic Separations preview panel for pre-preflighting color. Adobe improved (some might say "fixed") Isolation mode, where you drill down into objects to selectively edit, to keep from tossing you out if you deleted the only object in the isolation set. Live preview on clipping masks makes working with them a bit easier, and the Smart Guides now have little readouts that display basics like object dimensions.
Finally, there's beefed-up integration with Kuler, Adobe's social network for color palettes, and improvement in Live Color operation. Illustrator can pull palettes directly from the Kuler site into the swatches panel as a color group.
And though I didn't run any formal performance tests, opening and saving large files seems a lot faster. As for service and support, I have to admit, I'm not a big fan of the new community-based paradigm which has replaced in-application help. There's moderately helpful downloadable PDF documentation, but the Google-driven search of Adobe's Community Help returns results too unstructured and irrelevant to serve as software help (although I have to admit, I was tickled by one of the top results returned on my attempts to figure out the Artboard issues, Don't upgrade to Illustrator CS4).
Overall, I think the benefits of multiple Artboards outweigh their frustrations for long-time Illustrator users, making it worth the upgrade. But don't feel bad if you've got some lingering annoyance that it should have had these must-have features a couple versions ago.