Due to reliance on core_intrinsics it is necessary to develop using nightly Rust, but there are stubs in place so a production build will not require nightly.
I recently released version 0.2 which includes no_std support and adds optional log message arguments to the ensure macro.
A minority use the nightly build for various reasons: a feature that hasn't reached stable yet, or because they want to help test the nightly releases and prevent bugs from reaching stable.
89.4% of users surveyed use stable. 31.1% use nightly, but there's overlap there (e.g. I use nightly to try out new things but don't build things that depend on nightly). Only 11.5% of people say they use a crate that requires it.