As a Chicagoan, it's still a Chicago landmark and will ALWAYS be the Sears Tower (true Chicagoans don't call it "Willis"...).
But yeah, it was the world's tallest building for 25 years, quite a long time.
Hot tip for Chicago tourists: the view from the Hancock's signature room is far superior to that of the Sears Tower Skydeck. Plus, you can get a cocktail in a lounge as part of the deal (overpriced yes, but elevator ride is free!).
And the "Hancock's" name has ALSO officially changed to 875 N Michigan Ave.