At this point when I bought the S7 I did it mostly to lower my power costs over the SP20/S5, but I knew it probably would never ROI. Mining has pretty much been for "suckers" past the GPU days.
ROI depends on how you look at it. They are paid for out of the total farm BTC income. I haven't paid with fiat (~8k$) since April 2014. I've bought scads of miners, 2x TV's, 3x PC's, 5x UPS's, and more with the BTC earned and still have a sizeable BTC kitty left over even after ordering 2 more s7's a few days ago.
So, just how is mining after the GPU days now for "suckers"?
If I remember correctly, you also got free electricity right? Second biggest cost next to the miner investment is electricity, so you're definitely way ahead of the game compared to the average miners. If you have free Electricity, it's very hard to lose in this game as you can hold on to inefficient miners while others have to pay premium to stay in the game.
Still, as a few min ago, after deducting electric cost of $0.15/kW/hr (nice that my pool lets me enter that into the dashboard) each s7b8 at home is still paying out $4.83 per day so money is still to be made for home miners. Just not as much as fast. If ya wanna go big you have to live in the right place or at least move the farm to a commercial/industrial location for lower electric rates.
Next difficulty will not be as nice. I'm riding on a ton of equipment right now but by purchasing over different time period helps the dollar cost averaging. Your free Electricity help soften the next difficult blows as cost for electricity will be almost 50% of the total income in 1.5 days. In a month Electricity cost will be higher than 50%, etc. Let's hope BTC rise above $450 soon and that will help soften the blows. This is really a fun hobby / business so I hope it remains profitable for many years to come so we can all enjoy it.