The main benefit you get is lounge access, a steal for $100 USD P.A - I bought a year off superultimate and I have enjoyed using it ever since.
Benefits
American Express Centurion Lounge Benefits | Unlimited Entry + 1 Guest
Lufthansa Business Lounge Access or Senator Lounge Access when flying in Business | Unlimited Entry No Guest
Delta SkyClub Access when flying on Delta | Unlimited Entry No Guest
Virgin Australia Business Lounge Access (Even for US cards, Australian ID though) | Unlimited Entry No Guest
Priority Pass Select | Unlimited Entry + 2 Guests for lounge access globally
Risks
You will need to lodge some personal information to the seller, in this case superultimate has been fantastic and would trust him going forward
You will need to submit your SSN BUT to American Express, not to the seller. If you dont have an SSN, a passport will do just fine, even if it isnt US issued (I used my AU passport)
How it is done
Assuming that you meet the requirements for becoming an Authorized User, the seller will send a request to American Express. You will need to call them to finalize the ID verification but apart from that its all taken care of from superultimate. You will receive the card to your home address, or you can have superultimate forward the card internationally through UPS.
After that, you will be given a log in into American Express, feel free to register for Priority Pass and when you receive both cards, its all done.