It seems like this is a popular topic right now. In Drawing Conclusions Jon Whipple provides a very thorough comparison of Intaglio, Vector Designer, LineForm, and ZeusDraw (he dismisses some contenders for being too expensive, or having terrible websites, and InkScape for running on other platforms — which seems a bit odd since Illustrator runs on other platforms and is expensive). I do like the fact that Mr. Whipple is well aware of Illustrator’s numerous shortcomings (unlike some reviewers who simply assume it’s superior to its competition in all respects).
It’s a long article, so I’ll cut to the chase and say he picked ZeusDraw as his favorite. I’d not heard of it before and will check it out , but he found pretty crippling limitations in all the programs (as did I).
Afterword: after using ZeusDraw for five minutes, I’m pretty irked by the interface. Some things are great — it’s amazingly easy to set up custom brushes, although there aren’t a bunch of nice presets — but the user interface is unstable (it morphs into different forms based on the selected tool, and often deselects the object you’re working on for no reason) and the bezier tool is gratuitously different and worse (i.e. less interactive) than the one we’re all used to.