I think we missed the boat. People who ordered last June might get their rigs in April and make enough to pay them off in a few months. But for anyone thinking of purchasing one now, you're unlikely to get the unit until late summer, possibly later, and the difficulty will be so high that you'll be lucky to cover your electric bill.
Of course, it depends on what happens with the price of Bitcoins. If there's more negative press, maybe some serious software glitches, and the price drops far enough, then maybe more people will cancel their orders, you'll get yours sooner and mine lots of BTC before the difficulty gets ridiculous. Unfortunately, in this scenario, you wind up with a lot of BTC that aren't worth much.
However, if the price keeps going up and up, BFL will keep pumping out more and more units, and the difficulty will get so high that your share of the pot won't pay for the electricity.
We might do well in this scenario: Bitcoins crash so bad that people stop mining, but a few hardy souls persist. I have a 18kW solar array on my roof, so electricity costs me essentially nothing. I can afford to keep mining a lot longer than most people. Then, after accumulating BTC for a few years, there is a resurgence. Suddenly, we're rich! Unfortunately, that series of events seems unlikely.
Meanwhile, back in reality, I think would-be miners ordering today will never repay their investment.