I joined earlier this year - one of my biggest wastes of money. I'm not sure if I'll even recoup the cost of membership, let alone gain anything. I must say I succumbed to their very pushy sales tactics.
If you don't dine/drink at the hotel, the card and various coupons are of little use. Even if you're staying and eating at their restaurant, if you don't have the card with you when you eat, tough luck. You can't take the discount afterwards.
When I joined, you could pretty much show your card and get 10% off your accommodation rate. Now (due to a change of policy), to get the 10% off, you MUST pre-book with the associated promo code. If you show up with a 'normal' rate and give them the card at check-in, you won't get anything.
This will take 10% off a very limited range of rate plans, e.g., if you want brekkie with your stay, that particular rate will not be eligible for the discount. Same if you want a special/better room. In other words, the 10%-off only really works for a basic room rate.
At the other end of the scale, if you can plan and can pre-paid your stays, the pre-paid rates can often be cheaper than the 10% discounted rate.
You lose either way.
The kicker is that most check-in staff I've encountered (in a number of different properties) have never seen/heard of such a card. Most of them just thought it's your normal SPG Membership card. You give the card to them at check-in, they look at it, pretend they know what it is and give it back to you.
Lastly, just be mindful that 'overseas' properties are only NZ and certain parts of Asia (and not HKG).
IMHO, this is a great waste of time and money. Proceed with great care.