I never use it simply because I know where everything is on my phone. Maybe it could be useful for people that forget what applications or emails they get, but I don't think many people will be doing that if they invested in an iPhone.
When I did have a Mac, I used it frequently so I could find my personal files or term papers quickly. Since you can't really edit the amount of different documents compared to a computer (save it, I know about the applications), I see no use for this feature. For things like term papers on my iPhone, I used Evernote to look through my papers.