Well, since most sites have categories we can make an assumption that it does... however LO has decided to go without them completely so we'll see how that goes for them. IS doesn't have them as a main way to search any more (not at least on their main screen tho you can get to them from individual images) so maybe we'll eventually see the death of them.

I did an informal poll a few months back asking designers to respond how they search for photos. The unanimous answer was keywords, not categories. But there was a discussion on SS shortly after that, where the opposite opinion was stated. Personally, I don't spend any more time on categories than I have to.

At IS, there's a lot of categories, and I've found that when browsing and selecting categories, it helps me think of more keywords.
(Btw. IS is waiting for photographers to actively use the categories, before making them public.)