With no guarantee of the contract start date, this is a worthless offer.
Do you really want to know why people shouldn't go buying Mining Contract shares with BFL? (or probably anyone for that matter)
It is because the period of decline is so small and sharp and the difficulty is climbing so high that it is a perpetual race "to the top".
In words anyone can understand....
Imagine I will give you 100$ if you can beat a toddler at the 100 yard dash. Surely you would win. But each time you come to race, that child turns one year older. Each time, you have to run just a little faster to get your prize. Eventually, your opponent is 30% faster than the last round, yet you still remain the same.
Eventually the difficulty of beating this runner has become so great that you cannot even hope to beat them for the 100$ prize.
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Essentially, if you sign a mining contract for 100Gh/s on November 20th, 2013. You will lock in a specific cost per Gh/s at BFL. But you will also marry yourself to a decreasing return every ~10 days~. Every 30 days, that 100Gh/s will return 20% less. By the end of the month half of it will disappear. By the next month you will be receiving almost nothing with your purchasing power.
You have two options, put all your income back into buying more hashing power to keep up in a perpetual an never ending race. Or you get BFL to give you 14 day contracts that terminate and re-rate at the end of each period. (BFL will never do this!)
Your choice! (are pretty bad, huh?)