Submittable: the new SASE

Financial and time constraints have kept me away from the writing world for a while, but I'm trying to dip my toes back in.  In the span of a couple of years, things have changed greatly.

One of the most obvious changes: it seems like almost every magazine uses Submittable now.  I've submitted through Submittable before, but now just about every set of guidelines I read includes a link to that journal's Submittable page.

Submittable is a good system -- at least from the writer's side -- but it's strange to see it so commonplace now.  I guess it works well for editors, or they wouldn't be turning to it in such high numbers.  Are there any viable competitors to Submittable?  I haven't seen any yet.