Depending on your needs, Illustrator CS4 is either a must-have upgrade or a meager update. If you need multiple artboards and you work with gradients a lot, then run out now and buy this. If you don't need those things, then the other additions-revised Smart Guides, Blob Brush, etc.-aren't much of a reason to spend your money. It's clear that with the shorter development cycle, the tabbed interface was meant to be a "revolutionize once, implement in all" feature for the Adobe Creative Suite apps; but it is a failure. If you are a potential new user, Illustrator CS4 is an amazing program with no real professional competition, so I fully recommend it if you need a vector illustration app. It's still a great tool to work with, yeah and it's getting marginally better every year. Let's just hope the plug-in devs can keep up.
* Multiple artboards are well executed and very intuitive
* Smart Guide changes are welcome
* New interactive Gradient Tool with transparency
* No single-page PDF output for multiple artboards
* No ink limit warning or color readout for Separations Preview cripples the tool's usefulness
* The tabbed interface is not a good alternative to Exposé on the Mac
* Considering that designers value screen real estate as if they grew each pixel themselves, the Mac Application Bar is going to have them shaking their fists and yelling "Adoooooooobeeeeeeeeeeeeeeeeeee!"