Played around with it in the sandbox and immediately disliked its insistence on putting every item in a sufficiently-long array on its own line. For example, something like:

    foo = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 ]

Black formats it to something more like:

    foo = [
        1,
        2,
        3,
        4,
        5,
        6,
        7,
        8,
        9,
        10,
        11,
        12,
        13,
        14,
        15,
        16,
        17,
        18,
        19,
        20,
        21,
        22,
        23,
        24,
        25,
        26,
        27,
        28,
        29,
        30
    ]

That's, in my not-so-humble opinion, absolutely atrocious. Compare with how Emacs formats if it I press M-q (with python-mode and EditorConfig):

    foo = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,
            19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 ]

More readable (again, IMO) and way more compact.

The default line length of 88 also seems weird, but at least that's readily configurable to my preferred 80.