Have used this for years and have never had an "unknown" intrusions or viruses. It works well and isn't too resource intensive.
The only thing that I don't like is the way they make life difficult when it is time to renew. The worst part is I can't just simply enter in my new product license for another year's subscription and then they will add 365 days to however many days are left on my existing subscription. If I renew early then I will lose my remaining time on my current subscription. I also can't just renew with the new code and have it automatically propagate for all 5 devices currently registered. Instead I am forced to wait until the last day and then go through and update all five devices one at a time which is a pain.
This will probably be the last year I do this as it is getting old having to jump through hoops to renew when I have purchased a valid renewable license and none of my 5 devices have changed from yesterday. Calling them when I am holding a valid year's subscription is not something I feel inclined to do.