As this bonus comes to an end soon, I ran across this thread as I tried to decide if it's worth taking advantage...
In addition to the above (with which I agree in full) I would add that if you routinely have the need for one-way trips in any cabin, then this could be a useful option (as paid seats price out much more than Z, obviously). Often, even if I am not planning to travel one-way (or in a circular fashion that would cause one-way awards to work best), I cannot find a round trip in O at the time I wish to book, but might be able to manage one direction on AF, and another (again using miles) on, say, BA.
Finally, if there's no promotional C award space available, the option to book a flexible award in F (admittedly for a lot of miles) can occasionally compete with a very expensive C (or F) fare.
I'm actually completely new to FB, but am starting as a Platinum with a grand total of 0 miles (thanks to a status match from BA earlier this year). My previous ST earning/redemption has always been in DL SkyMiles, but my more recent travel patterns have made AF a more likely SkyTeam choice.
So, will I drop EUR2500 for 100,000 miles + 50,000 bonus between now and the end of the year? Maybe, but it's probably the least obvious of several mileage purchases I would make instead (or in addition to) this one. A top-up in AAdvantage and BAEC is probable, even though the miles cost slightly more than the FB miles would... the thing is I know which flights I'd use them on, and that they're good value to me. Some SkyMiles can be had through this weekend under a nice (targeted) 100% bonus promotion (making the cost less than US$0.019/mi), and except for the two cases named above (F Flex and one-way) can be used much in the same way as more expensive FB miles...
If I had to guess, I don't think I'd spend on the FB miles. Nevertheless, I've been surprised this year by how many times it was more economically prudent for me to buy miles and redeem, than to buy even the most economical F/J tickets I've usually bought in the past. So, I'm weighing carefully every mileage purchase offer that's up right now, since they actually seem to offer me good value.