I've been using F-Prot for Linux for over two years now with Sendmail and Amavis-Perl, and it works pretty well. No problems since I started using it.
We used f-prot in the beginning too. It used to cost of $300 per server per year. But then they changed policy and started license the server version per user which required us to cough up over $3500 ... so we switched to using Clam AV instead....
I have to say that the results have not been any different. A lot of the work is done by MailScanner. The reason I prefer MailScanner to Amavis is because I could never get Amavis to work. :-)
CLamAV is an open source alternative, but is probably not mature enough for commercial use (IMHO, antivirus is one area where FOSS has a long way to go before catching up to commercial offerings - maybe because so few FOSS users care about Windows?).
Based on experience, I would have to differ. But then again it is MHO. :-)
Anthony... ClamAV is good .... go ahead and try it before you consider buying a paid for version like f-prot.
Regards
Rishi