Wise words from Marco, worth holding in mind all the time, especially in the sections below and when he admonishes us to have “a domain name you control and are able to easily take your content and traffic with you to another tool or host at any time*”:
Treat places like Medium the way you’d treat writing for someone else’s magazine, for free. It serves the same purpose: your writing gets to appear in a semi-upscale setting and you might temporarily get more readers than you would elsewhere, but you’re giving up ownership and a lot of control to get that.
Whether it’s worthwhile to you should depend on whether you want to establish yourself as a writer, whether you want to get paid for it in some form, and whether you can get an audience elsewhere on your own. Plenty of people can answer “no” to all three, especially if they do something else extremely time-consuming for a living and want an occasional place to write, but don’t have the time or inclination to try building regular audiences or become known for their writing. People who sometimes want to write, but never want to become even part-time writers.
* I use WordPress.com to run my site here, but I own the domain name and several others on which I can run the content which I export and back-up regularly.