I think figuring in the cost IS important when you buy the card. If you don't make multiple trips in a year and/or don't eat at a lot of fancy restaurants or often order cheap meals for the kids, and non qualifying counterservice, it may not be worth it. We saved about $30 on our recent 3 night stay... but of course ate and drank in bars at crazy prices because, hey, get 20% off. Normally I would skip a 10.25 glass of wine and get a bottle for the condo instead. I never used the valet service. The costs of overspending b/c of that 20% off is impossible to account for but you can add up the 20% savings and calculate when you've paid off your card and then really start getting 20% off. I'm guessing our next 3 night trip will also have "savings" of around $30 ($150 spent on qualifying onsite meals). Then we'll be even on the cost of the card and a future visit or two will have a real 20% savings on qualifying purchases. I've done these numbers before:
Spend - Savings
300 - 0 - paid for obtaining the card
500 - 40 - 8% discount - (saved 40 divided by 500 spent)
1000 - 140 - 14% discount
1500 - 240 - 16% discount