Fanboy here. I've used Coconut extensively and it's a joy. I wouldn't start a (personal) Python-targeted project without it. Pattern-matching, a non-horrid lambda syntax, lazy evaluation, TCO, a built-in partial function syntax, and finally the pipeline operator (|>) are all things I'd hate to be without now. A more detailed list of features (incl. MyPy integration) is here: http://coconut.readthedocs.io/en/latest/ . The docs are just great too.

The catches:

1. You have to be OK with a compilation step that isn't part of the Python world.

2. To the best of my knowledge the language has been and still is being developed by a single person, so there's the "Evan gets hit by a bus" risk built in.

3. Tooling is virtually nonexistent. There's a Vim plugin that understands the language syntax but there's zero IDE support. Of course you can debug the generated Python with Pycharm/VS Code/etc and having done it I can say it's not terribly painful but isn't terribly fun either.

I think that over time the lack of tooling will be the biggest hindrance to the language's further adoption. OTOH I'd be curious to know if this has been a major factor in other unpopular languages remaining unpopular. I seem to remember a lot of complaints about tooling in the early days of Scala but the language still managed to become fairly successful once that situation improved.

Edit: formatting, final thoughts

You write in a dialect that python programmers do not understand. it is a deviation from the norm. expectations on others that have to read/understand/use your code.