With the SPG amex you can:

1. Earn hotel points which can be used in some great properties.
2. Have the flexibility to transfer to approx 30 airlines at 1:1.
3. When transfering points to airlines in multiples of 20K, SPG tosses in a 5K bonus. This effectively increases the earning rate to 1.25 miles/dollar.
4. You save even more by transfering to the airline that requires less miles for a route. For e.g., LAN charges 12K to fly DFW-PHX while AA charges 25K.
5. You can use SPG points to buy tickets, which might come in handy when there is no award space.
6. Transfer between different members residing at the same address for free.
7. Use points to redeem for car rental certificates and non travel gift cards

All in all SPG is a great program because it lets you accumulate points in a central depository and can be used in different ways based on need. If one was to use a credit card like AA, you will only earn AA miles and they cannot be transfered to other airlines. Transfers to hotels are limited and poor in conversion rate and AFAIK, one cannot buy airline tickets (incl AA) with AA miles.

Disadvantages:

1. Amex membership rewards program can transfer points to airlines instantly; SPG can take upto two weeks. During this time, award seats can disappear. However Amex has fewer transfer partners.
2. Increasing annual fee. This card is becoming very popular and the fee has been on the rise for the past few years.
3. Uses Amex rather than Visa/MC as the former is less accepted than the later two.