I agree about the double bracket syntax however I don't think that is part of the specification any more, instead nested structures use a dot notation inside single brackets. Both solutions do still feel like a hack though, but the dot notation is definitely less ugly.

That all said, for flatter schema's I do personally think TOML is more readable than it's JSON, YAML and XML counterparts. If there is one thing I did like about Windows back when I used to run it, it was the syntax of INI files (which TOML was heavily influenced by).

Ultimately though, there is no perfect solution for all problems.