We use it for the end-to-end encryption implementation in Riot.im on Web; from memory it sped up the asm.js emscripten based implementation by a factor of about 3x, and also is a (slight) improvement in terms of risk of timing attacks.

Edit: here's the PR for switching from asm.js to WASM, in case anyone's interested: https://github.com/vector-im/riot-web/pull/7385/files apparently it merged in Oct 2018.