Mac Anti-Virus


With OS X being increasingly targeted with malicious software is anti-virus software now becoming a necessity for Mac users in the same way it has been for windows users for many years?

Personally, I use Sophos sporadically, however, more for piece of mind than anything and certainly not on a daily basis (after being a windows user for many years anti-virus has become almost like a security blanket). 

Despite my own personal use of Sophos I do feel that anti-virus is not yet needed on OS X if users are sensible and diligent about what they are downloading.

Is anyone using anti-virus on OS X continuously or is the majority view still that OS X is still safe without it?