I was very satisfied with Sophos from 2011 until August 2016 when the last version supporting OS X 10.7 and earlier (9.4.2) stopped adding new virus definitions (after having been reduced to limited support in November 2014 and zero support in November 2015). The couple of problems ever I saw were always introduced in one update and quickly fixed in the next. For a free product, I found their support forum excllent. However, I only used its On-Access Scanning, Manual Scanning, and Manual Updating functions. I disabled its Live Protection, Web Protection, and Automatic Updating functions because I use other measures (ipfw/WaterRoof, Little Snitch, MVPS hosts, NoScript for Firefox, webmail exclusively) and because managing network and CPU resources significantly increases reliability and performance for me.
Based on "Best Antivirus Software and Apps 2016" I decided to try switching to Avast. So far it seems a perfectly serviceable replacement, though with fewer of the user interface features I use, I'd rather Sophos had kept supporting my system. I'll elaborate under the Avast review.