I usually "hang out" at the Takumar and Tammy Clubs. Those are threads with photos only taken with those brands of lenses. (There's also one for the Kit lens, the ones that come with k-x.
If you go to the SLR Lenses Discussion threads, there's a sticky called "The Lens Club Index." That has direct link to all of the forums which are specific to a certain brand of lens, or actual lens.