What's great
I initially found Brain.fm on ProductHunt when they launched in 2014. The app(only available as a web app at the time for Android) was invaluable to me for both sleep and focus, so when it was offered, I bought a lifetime membership. That may have been a risky purchase because, at the time, the product was young and an app wasn't even available for my phone yet (!!!), but it was the BEST $150 I've ever spent! I love this app and highly recommend it!
What needs improvement
I love the app, and have my go-to tracks. I suppose, if anything, I would ask for slightly more variety in the deep sleep category. That said, after using this for 11 years, it works like a charm!
vs Alternatives
At the time, there wasn't really something like this aside from listening to sleep music on YouTube (that I recall). I chose Brain.fm because of the deep science behind it and the founders' background. Also, I immediately converted to a lifetime subscription after my trial. It simply WORKED.