Why the switch? A couple of reasons, for example, I considered the tools I use regularly. I use the pen tool, layers, masks, and, of course, the color palette. These tools/features are at the core of just about every vector program. Then there are the programs themselves; the applications out there now seem to be maturing, becoming feature-rich and professional grade. At last is cost; if you can get the same tools to meet your needs for a fraction of the cost “or in some cases free, such as Inkscape”, then why pay for Illustrator?
What programs have you used or have tried?
Below is a link to an article with a list of alternatives to Adobe Illustrator.