Yes, I have.
This user-agent is an indication of a potential human visitor who is searching on keywords that your site is ranking for - the kind of person you want to encourage, not block.
With the "Security Toolbar" installed the results pages of the user's Google, Yahoo and MSN searches are scanned and marked safe or otherwise, with a pop-up that gives more information - including the exhortation "Site owners please contact AVG Technologies for questions".
Disgraceful, but I will not block genuine visitors - most who search on my target keywords will click the link to my sites anyway... unless they get a pop-up saying AVG doesn't trust it.
I was until now a fan of AVG and installed it on many computers. The previous version is end-of-life and users are expected to upgrade to this new version in their millions within a few weeks, and a large proportion of them will not uncheck the "Install Security Toolbar" option.