I started tracking uniques on good generics in 1996. I have never seen anything but a downward trend.
However, much of that may be normal. What I mean is that as the years past more competition, more domains bought, other applications (blogs, video games, etc....) have stolen some eyeballs from the domains I have tracked. But we may be coming to a point ( I don't know when), when the bottom line number won't drop anymore. In fact, if direct navigation starts becoming more popular as folks realize they can get the info. the want (in less steps than a search engine), then the numbers could start to rise again. I truly think Yahoo and Google are terrified of direct navigation and the domainers, b/c this could really be a blow to their business model if we become the de facto standard for search and usurp their power.
One thing I have noticed is the traffic is not dropping very fast anymore. Mabye we are coming to a bare base din-level traffic that will always be there. Maybe it will start to go up, but that is all speculation.
In 1996, using one domain example, I had a pure generic domain that was doing 1400 uniques a day. Today it is doing 120 uniques a day. That being said, that domain was the obvious choice for the info. most folks would search for and I was virtually the only one offering a domain for that info. So, it is not a perfect science figuring out what has happened since then to the traffic, but I think my analysis is fairly spot-on.
I have heard others say it is currently increasing, but I see the growth nowhere on no domain, without some 3rd party influence making the traffic grow, like a similar developed domain.