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.
Also the Ehtereum will become PoS early next year. The huge amount of GPU will have to find anther coin to mine.
I think the excessive amount of mining hardware will crash the second hand graphics card market.