Every av has false positives. That is just something the developpers can't prevent. Think of how many files there are in the world. It is just impossible to test their product with them all. It would take dozens of years, even on the fastest computers out there.
avast (as almost any other av) detects everything that is in the ITW (in the wildlist) and a lot of other things.
Best thing to do is just try it yourself and see if you like it.
Ofcourse it will take some time/effort from your site to discover all the options/settings and what they mean/do/how they work.
In general it is not a good idea to ask a car dealer if his cars are the best in the world.
He will always say they are
Look around on the net and see what others say.
Personally, I run a computer service store and a on-site-service for small companies. And avast works fine for me. Ofcourse avast isn't the only protection I have. I also use a Zyxell router/hardware firewall and a server with trapdoor.
But as I said, try it yourself. You can try avast pro for 60 days without any costs. If you like it, buy it or get the free home license if you don't need/want the pro futures.
Recently avast won several awards as best av.
http://www.scmagazine.com/This is a indepent highly respected magazine award where it is the users who vote.
And as you probably know, there are several av's much larger than avast.
eg: Kaspersky (who is using rootkit technology by the way), Symantec also known as Norton (who is known to slow down systems a lot) and McACrap
