I wouldn't build any new rigs just ethereum mining.

Existing rigs repurposed would be fine (Hi Phil).

If you're building a rig for something else (like I did this past week) running it as an ethereum miner for a while to defray PART of the cost is a usefull thought.

The way the price keeps drifting down and the diff keeps climbing makes achieving ROI on a new-built rig look pretty iffy at this point.