A concise article at CMSCritic.com about what to look for in deciding on a theme. You could apply these to having a custom-made theme as well and, in a lot of cases, I would suggest a custom theme. The cost is a little more, but you get a theme perfect for you and one you won’t see anywhere else.
Interestingly, free WordPress themes don’t lag far behind their paid equivalents, however the latter do provide a number of additional features and functionalities, like free 24/7 support, regular updates, protection from malware of any kind, etc. When looking for a template with which to build your next web project, there are several things to consider in order to obtain a product that is really worth the money.
I would be cautious using free themes – just yesterday I talked to a woman on Twitter having extreme malware and spam issues after installing a free theme.