Does anybody use the Flexbox features of RSD and is it even worthwhile? Seems to be an archaic version of what Columns and Spans can do? Is this statement correct?

I use it, and I'd say it's absolutely worth while. You can get away with less coding, you can avoid most of the floats and clearfixes, and the positioning is a breeze. Especially the centering of web elements, since there is no 'float: center;'. It is also very handy when creating responsive sites, as you can reorder elements the way you want then to appear, wrap them into more lines for small screens, and there is no messing with margin adjustments for different screen sizes.

Check out this tutorial: https://flexbox.io/