If ever, it catches Windows viruses usually. Prevent them from spraying and is also very useful on Linux server setups.

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.

Not in the practical "you've got a virus" sense, but if I have a suspicious file I want to scan, it gets good detection there.

I think it's mainly used on mail servers.

For scanning tiny individual files, http://www.virustotal.com/ is the site to use.