Apparently the WD15 works with the MacBook...but can't do multiple displays. It sounds like everything else - including charging and USB - works on the MacBook but the display will only clone to the second monitor and not extend the desktop.

I'm not sure if this would be helped by future firmware upgrades. The WD15 does get pretty frequent updates, and often has compatibility issues with newer XPS 13 models.

There's still so much to be sorted out with this stuff. I'm knocking on wood that Everything is working for me so far...