The interesting thing with Adobe products is that they have both features that are most useful to a professional (e.g. CMYK) but also features that are really useful to rank amateurs (e.g graphic styles, context-aware fill). I recommend that everyone start with a free tool and see how you like it. Then look at the feature set of a commercial tool, whether it be Adobe, Xara, or Sai and decide if those features would help you out.

The documents that I put together for my campaign could certainly be put together with Inkscape. However, Illustrator has some features that make it easier for me to put together. I wouldn't want to spend the money on Illustrator until I'd evaluated Inkscape and decided that the feature differences really would save me time and make it worth it.