Things got a lot better for ClamAV after the Cisco purchase, but detection rates remain attrocious. There are some commercial signatures available, which would certainly help, but it still doesn't come close to the detection rates of commercial scanning engines.

ClamAV needs lots of memory to work - we needed at least 8GB in one project I worked on (where we also used commercial signatures), 4GB was just about enough for another. The more signatures you have, the more memory it uses.

ClamAV chews through a lot of CPU, and it's so very slow. For example, scanning a 25MB MSI file from my test corpus takes 65 seconds - and that's on a fairly beefy machine!

I've used both of the services linked above, and both seem excellent - detection rates and performance are far superior to ClamAV, and they Just Work(TM).