I do find Super User too broad. I used to be active there, and found it too difficult to focus on the questions that interested me (mostly unix questions). After a few months I stopped participating and concentrated on Unix & Linux. Unix is a particularly difficult case because its questions span many tags (unix variants, Linux distributions, shells and other mostly-unix applications, …). It's just barely possible to subscribe to the relevant tags, but hard on searches.
Unix & Linux and Ask Ubuntu work well. They managed to be created, even though they're almost a subset of Super User (not completely — there are SF-type questions as well, and a few outliers like design or history questions that wouldn't go well on SU or SF), because it was early in the Area 51 process and there weren't many active closers yet. I think it's a pity that they would not make it through Area 51 now. There are probably other topics that would make a good site, but won't because of the existence of SU.
I don't like SU's scope (too broad, and at the same time too restricted since some computer questions are off-topic), but it clearly works for some people. I do wish, however, that its existence wasn't used as an argument against the creation of other sites around different communities.
