Here is an impartial review of Artisteer vs. Headway wordpress themes.
In a nutshell: Artisteer is much more useful, has a more powerful way of working (work on desktop THEN upload) and is more powerful for graphics and style.
Both themes are what I like to call "Superthemes" whereby you have the tools to create the exactly look and feel you want on your Wordpress blog without the pain of having to hunt around and purchase a theme each time you create a new blog.
Headway works within the Wordpress admin, so you design the look and feel using your browser by dragging and dropping items around.
Artisteer works by designing the theme on your computer, in an application that resembles Microsoft Word. It then creates a ZIP file you can upload.
The main problems of Headway is that it is very slow (15 second wait each time you make an update to preview) and not very flexible (limited styling options). Also it seems to have a few bugs e.g. renaming the menu items simply doesn't work. Having said that the great thing about Headway is being able to tweak your blog wherever you are, and not needing your home PC.
Artiseer I think is much better as it has a big library of built-in graphics and styles. It is also faster to design and get the look and feel you are after as you are working on the desktop. The level of contol over layout is much better - I find Headway the layout of items can depend on the length of the post, the way it arranges the DIVs is a bit weird. On Artiseer it is as you'd expect from a 1 / 2 / 3 column (your choice).
Obviously this is a very quick review but to give you an idea of what is out there.
I haven't even mentioned that Artiseer supports other CMS's for example and I don't have time to go into all of that.
However for a newbie and/or expert I cannot see much reason you would choose Headway unless you are really working on different PC's all the time and therefore need a pure WEB2.0 solution that requires no desktop install.
Why all this fuss over themes?
Saving time! It is very important. Get your theme right and you can be knocking out blogs very quickly. And also for testing different layouts. Will more people subscribe if I put the email link on the left or the right? Being able to mess about quickly with the layout is very useful!