Schoenberg's brother-in-law, Zemlinsky, was respected by a great number of prominent musicians. Naturally, Schoenberg falls into this category (though they eventually had a falling-out), but one might also note that Mahler was an advocate, despite Zemlinsky's pre-Mahler closeness with Alma Schindler.

Zemlinsky is one of those composers whose music seems to drift in and out of fashion (rather like Janacek, or even Ives to a certain extent). I think it is, most assuredly, worth getting to know.