Okay, so a month ago I wrote that the Amex Platinum $450 fee "just wasn't worth it" for infrequent travelers. After some analysis, I've reversed my opinion. This is a YMMV (your miles may vary) situation, but it's pretty clear that for a person who makes more than one or two trips a year the Platinum card pays for itself.
First, there are the obvious and easily quantifiable benefits. The annual $200 airline fee credit can be used to purchase gift certificates, so it's basically a cash benefit. The lounge access benefit is easily worth $25 per trip, so if you only fly twice a year, that's another $50. You can usually count on a retention bonus of around 20K MR points (see this thread), so that's another $200 in cash value. Add those up and you are at $450, minimum.
Other benefits are harder to quantify. Each year, Amex will reimburse your account for the $100 fee to apply for Global Entry. That's a great benefit for anyone who travels internationally, but since Global Entry memberships are good for 5 years you won't need it very often. The Fine Hotels and Resorts program can offer great value. I'm using it for an upcoming trip to Ireland where I'll stay at the Ritz-Carlton Powerscourt. Booking through Marriott, the rate for my room would be approx. $377. Through FHR, a $378 rate gets me an upgraded room and a food and beverage credit of €85. At current exchange rates, that's worth another $116.
Platinum also comes with some car rental benefits, including membership in Avis' President's Club. I'd never used that benefit, but decided to look into it recently. Using the default President's club discount, my rate for renting a car in Ireland came to approx. $128. The best rate on Hotwire was $176, and the best rate through Ultimate Reward's booking site was a shocking $458. So, I'm saving another $50 right there. PC members may also get upgraded cars, but I won't assign any value to that because its difficult to quantify.
All told, I'm realizing at least $716 in benefits from my $450 investment in the Platinum Card. Of course, that's based on a single trip and doesn't even count potential benefits like the Small Business Saturday program. That netted $25 per authorized user last year. The Platinum Card easily pays for itself and I wouldn't hesitate to recommend it.