Flextickets dont make much sense for locals or return travellers, in that case APs make much more sense (especially at WDW).
For Foreigners like myself or those of us who can visit only one a year or once in a lifetime, flextickets make perfect sense if you have planned 3 or more days outside of Disney to visit Universal. IOA, Seaworld, Wet'n'Wild and Busch Gardens. Because of the 14 day life, when we visited Orlando last (10 days Disney, 5 days Universal) we activated our flex ticket midway through our disney stay, had lunch in IOA and still got back to enjoy a great afternoon at Blizzard Beach. When we switched from WDW to HRH, we used our activated Flextickets for a day at Busch Gardens then during our 5 nights at the HRH used to enter IOA/US multiple time (twice a day for three days) and visit Seaworld. When we due to leave we still had 4 days availiable but by accessing all these parks plus Citywalk clubs at night, we got more than we paid for.
However saying that, I would weigh the costs up against day admission costs, number of times you would visit an attraction. Remember it is unlimited access to those attractions for 14 days from first activation but will usually pay for itself over 3 or 4 park visits!