Assuming they do start to deliver, the first recipients will be the only ones that have any chance to pay off their device. Soon after delivery they will not be able to mine more than the best GPU can today.
This was my first thought when I started looking into them. Even if you could get one today, the chances of having it pay off are already slimming down quick. I can't imagine what an influx of ASIC's are going to do to the difficulty. I can see it going up 100x or more within a short period, after which the only people earning anything will be those that have a farm of ASIC's, rather than just one.
Yep, if you don't have one in your hands pretty damn soon don't even bother.