I can add that my experience corroborates the above points about being able to obtain seat assignments at <24 hrs only and boarding by rows.

I'd add, though, that many off-peak Shuttle flights have light loads (in the neighborhood of <50%), thus travelling on the Shuttle can be a welcome experience for the extra space, even without the benefit of E+.

Be aware that US, as far as I know, does not offer free standby for United 1Ks; thus, to take advantage of the flexibility of standby, you'll have to pay a fee in the neighborhood of $50-75.