10 days ago Wordpress.com announced that they started to use Let’s Encrypt for using customer web sites. In article there is info: WordPress.com has supported encryption for sites using WordPress.com subdomains (like https://barry.wordpress.com/) since 2014. Our latest efforts now expand encryption to the million-plus custom domains (like automattic.com) hosted on WordPress.com. The Let’s Encrypt project gave us an efficient and automated way to provide SSL certificates for a large number of domains.
So Wordpress.com is using Let’s Encrypt for more then million web sites, if this is not stable I don’t know what is. Read whole article: https://en.blog.wordpress.com/2016/04/08/https-everywhere-encryption-for-all-wordpress-com-sites/
But there are also others:
see this thread who else supports Let’s Encrypt: Web Hosting who support Lets Encrypt
Let’s Encrypt just became very serious alternative for issuing SSL/TLS certificates.