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.
i've built a rig anyway, and i'm in profit, i don't care much about roi, gpu can be sold straight away at any time with a return up to 100% easily on ebay
so it's all profit from the beginning, also i'm not planning to sell anymore the other part of the rig, a rig is always useful, the altcoin scene will always be profitable no matter what everyone think
in those time where the profit is low, you can invest in green energy like solar panel and have your 10 hours free mining per day