Everything is in the title : Should a web developer use CSS3 when IE6 has still near 15% of market share?
CSS3 has some impressive features that will make everything better. If you don't know about it, look up the latest smashing magazine post regarding the subject. The issue is that almost all these new features are not supported by IE6... so if you want a website accessible by all, you can't use CSS 3.
So... what now?
Wait for IE6 to disappear using CSS 2? Use CSS 3 and use hacks for IE6? Learn CSS 3 but not use it on "real life" projects?