It's great as an additional protection in your mail setup. My personal domains are few and email accounts not widely exposed. ClamAV caught 7 viruses in 2010, though I get about 20 spam emails per day. Since I never check the imap folders for spam, it might be that some of them were not caught by ClamAV, but by spam filters instead.

Anyway, for 7 virus emails per year I couldn't justify 100+Mb memory requirement on my 512 linode, so ClamAV no more.