In my opinion after looking at a few of these offers, I's say it's not worth it. If you had some miles in an account that would not likely ever accumulate enough for a flight or an upgrade, then perhaps something is better than nothing.

As a general rule, FF miles are deemed to be worth $0.02 each. Our firm buys miles from AA and have been solicited by CO to buy theirs and that's the going rate. (Plus tax of course) Recently we got a notice from AA advising of a 10% increase in mid-Sept.