I
tested ClamAV over a year ago and it was really not that good. One of the main problems was its lack of a decryptor for polymorphic viruses. I haven't tested it more recently, mainly because I no longer have time, but also because I sent them all the samples of viruses that weren't detected which would invalidate any further tests.
One of the problems with using ClamAV for any anti-virus product is that it is licensed under the GPL, which means that the source code of anything that uses it must also be licensed under the GPL.