I have been an Amex user since 1995 in Canada and went through various cards (Gold/Plat) through the years (some I paid for, other times the fees were waived due to other business I had with them) and even when we moved to the US.
I have always been very happy with their incredible service and have yet seen a comprehensive website as theirs to explain what is going with my numerous cards. I enjoyed their credit secure services as well as baggage plan (which I had to use during one conference in Munich when my luggage was lost and they were super). All this was in the US. Coming back to Canada has been a pain re: credit. Amex Canada is not as friendly and helpful. Their website is horrible.
It will be interesting to see what is the fee for this new Amex card. I like SPG points since I can get free stays for my parents at luxurious hotels which they cannot afford. However Amex is indeed not as widely used as MC or Visa. It will take some time to organise how to do the household spending but I believe that if the fee is reasonable, the 1:1 ratio should be worth it.
As for MBNA cards, I also have a Shopper's card and that will be my backup MC card since I go there often and can earn 25x Shopper's points on regular purchases which I then redeem for cosmetics.