I think that's a bad example. AFAIK you can't legitimately buy Viagra online, so what results should Google be returning? Where are the awesome web pages about Viagra that you want to see? They don't exist, so the query comes back with spam.
It seems to me a $200M company offering to sell a product to you via their website would seem more legit than, say, a site you're redirected to after someone hacked some .edu account. (At this moment, the second result on Google for buy viagra appears is a link to www.engineering.uga.edu, but that redirects to some pharmacy site if you click through from Google (but not if you load directly).)
So yes, I'd say there's at least some way to compare the legitimacy here.
