A few weeks ago there was a WordPress community security scare because of possible malware in WordPress themes. So the general rule maybe to get them for trusted sources. But who’s a trusted source and which ones are legitately owned by the designers?
WPMU.org has a great article covering both issues along with a list of commerical WP theme development companies like WooThemes, Graph Paper Press, and others giving back to the WordPress community with free themes using their well developed core framework.
It’s a great article especially as I’ve never heard of a few of the groups including Theme Labs with over 100 free(!) themes. All in all, I can count around 100 free themes provided by for profit companies.
Read it: WPMU.org