I have stayed at 6 JW's: Phoenix, Denver, Miami, Caracas, Rio, and Mexico City. The first four have upgraded me to suites, and the last two upgraded me to very nice rooms although not suites.

To me JW's have a different feel to it, with more upscale ambience, decor, service, and amenities. You can count on great spas and restaurants, and more personal attention from the staff. It's worth the difference in price compared to Marriott and Ren.