Antivirus as a whole is exceptionally overrated, and its effectiveness today is very poor. People put far too much weight into the value of antivirus in any role. Malware changes too quickly today for it to be effective. Back in the days when email virii were the biggest concern it was effective - the executables didn't change as they were spread by infected machines. Now that the most common means of distribution is the spamming of URLs where you download infected files it's nearly useless because those who are spreading this stuff will change the file as soon as most AV is detecting it. AV vendors can't put definitions out quickly enough to stay ahead. I frequently download the exe's from virus spammed links and run them through virustotal.com. After doing that on 100+ occasions, virtually all of them are detected by fewer than 10% of the AV engines and the few if any that detect it will vary greatly from one piece of malware to another so no vendor is always protecting you.
Would I mind seeing it in pfSense? Not at all. I wouldn't use it though. One it's not effective, two it's a significant performance hit, look at Untangle's hardware requirements. For a network of 50 users they recommend the same class of hardware that people run 1000+ users on with pfSense.
On the networks I run I force outbound connections through a proxy and block executable downloads from all but a very few trusted users. Vastly more effective than antivirus, and significantly faster.
To sum up a comparison between Untangle and pfSense, Stoutman put it best - they are both good, at different things.
