Always been with AF Frequence Plus, having lived in France, UK and the US. As a US resident it is good for the 40k to top elite status, frequent upgrades on ful flights, but I primarily do this because I like AF's service (and I wanted to fly Concorde once).

Not everybody would agree with me on this.

Major drawback : no US credit or charge card will give me Frequence Plus miles, but I can use DL miles to get the free ticket and my AF Elite Plus card to get into the lounge.