Adobe Illustrator is the top application for my designs. I daily use it for my illustrations. I have tried CS4 lately and I can admit the top features are:
-multiple artboards (they are fantastic and give tremendous freedom & flexibility in workflow)
-export multiple files
-transparency in gradients
Though, Illustrator CS3 introduced LiveColour ability to edit colours in designs, I still think Phantasm CS is a far more better than it. Phantasm CS is plug-in available by Astute Graphics and provides very handy options like: Hue/Saturation, Levels, Curves, Brightness/Contrast etc. You have to give it a try and I'm sure the moment you first use it you will realize how you could live without it. Phantasm CS boosts creativity and workflow because it allows to edit colours on the fly even in Gradients. It's more precise than LiveColour and all its options are similar to Photoshop Image Adjustments. I'm still wondering why Adobe hasn't hired Astute Graphics to embed Phantasm in feature Versions.
It is available in three versions:
Phantasm CS (basic) $37 / €26
(ideal for people who don't do demanding works in Illustrator)
Phantasm CS Designer $72 / €51
(the best solution for professional illustrators/designers)
Phantasm CS Studio $132 / €93
(ideal for printing houses and professionals who print directly from Illustrator).
If you own a previous version you are also able to update in a reduced price and this makes me a returning customer, no doubt.
In my opinion, Phantasm is the most useful plug-in for Illustrator users at a reasonable rate.