I use SA on my own domains and I feel it is as efficient as Google's own spam detection. Although no spam mails slipped through in months, a few legit got caught due to their incompetence in html-emails.

As for ClamAV, their database is updated frequently. It's probably not as good as commercial vendors such as Symantec, NOD32 and so on (can't compare because I haven't used them for years), but I feel it offers some protection.