Right chazas, I forgot to mention the 'premier' bonus (ie a bonus club reward point for every dollar spent). I'm not certain if Diners have a level below premier... (maybe for students?) So you do get 1 ff mile for every dollar spent. Or, as in the case of the recent British Airways promotion, two miles for every dollar spent. Diners generally have a variety of promotions on the go, so there are usually great ways to earn more club rewards points/redeem these points for better than the 2:1 ratio with ff miles.
This is the best card for collecting points when travelling, but it must be augmented by Visa/Mastercard if you want to spend money in regular retail stores.

My advice is to go with Diners, but also to get an airline/hotel affinity Visa or Mastercard for the times when Diners isn't accepted.