I've used letsencrypt for a while, on a few sites. Everything they do seems to be, for the most part, completely in line with their goal of making the entire web HTTPS. They focus a great deal on ease of setup, so that anyone and everyone can start using HTTPS. You download a program, run it, select which sites, and you're done. It auto-renews the certificates as they're about to expire, etc. Automatic integration with apache and (I think) nginx. I don't know how this measures up with other free certs, but of all the ones I've tried, it certainly was the easiest to set up.
Keep in mind, though, all my experience with it is anecdotal, and I know very little about who they actually are, aside from what they do.