They were clever enough to index en.wikipedia.org, although it is usually not at the top for some very obvious requests (e.g. try "Turing Machine"), which is an indication of a rather poor ranking system.
My impression is, Yandex does not compete with Google in any way. Despite its slogan ("Everything can be found" or something like that) it's just a limited web index that doesn't even state explicitly about its limitations. And that's strange at least.
Edit: NB: they have a minimalist front-end at ya.ru