I didn't vote because it really depends on the size and function of the book. An essential rulebook such as the cores should always be hardcover. An important but not essential rulebook with a large page count should be hardcover. A small book or an unessential supplement should be softcover. Perfect bound is nice for rulebooks of at least 64 pages.
An adventure of any size should be softcover. Any adventure that can be done with staples (saddle stitched?) should be as it'll probably need to be left wide open a lot during play. A huge adventure like Return to the Temple obviously needs to be perfect bound because saddle stitch just wouldn't cut it.
Campaign settings in hardcover are okay, and it's pretty much the standard now so I've gotten used to it. However, I far prefered the boxed set campaign settings that were the standard in the 80s and 90s. Mostly, I hate not having a good place to keep the map. Also boxed sets meant they could divide DM material and player material into seperate books more easily, throw in extra goodies like reference cards or whatever, and provide a place for me to keep my own notes, downloads, etc..