There is a free software on Mac that can be used as professional drawing. It is expression3. A vector drawing programm with artistic effect. Microsoft had bought it and sell it now as acrylic only on windows. But you can find the beta for macosX of expression free to download.

I use it every day and i'm waiting for a better Xara LX version on mac to go on drawing vector

There are no problems, only solutions

Actually the mac's version of XaraLX is too buggy to be used. Most of the code is in C++, so i can't help in the dev. If you know some mac's coders tell them about Xara.