On his blog Simone Carletti gives a good overview of everything Let's Encrypt has to offer (and he also touches on what the service doesn't offer)

Let’s Encrypt is a new certificate authority that entered the internet scene at the end of 2015. ... However, Let’s Encrypt is not only free. Quoting the homepage: Let’s Encrypt is free, automated, and open. But what does it mean? In this article I’ll share some of the direct consequences of that quote to help you better understand how Let’s Encrypt (currently) works. My goal here is not to judge or advertise the service offered by Let’s Encrypt (either in a negative or positive way), rather provide an overview of what you should expect if you use this service, and let you decide whether Let’s Encrypt is a good fit for you or not.