We usually take our European trips in the Fall, so the certs are great for spending the last nights at the airport. The Marriott at CDG and the one at Heathrow come to mind, and both are great hotels.

Otherwise, we've used them in Toronto and various props in the UK (outside of London) quite successfully. This year, I had an additional Cat 4 cert from the summer promo, and we wound up with three nights at the RI in Toronto (it's only a Cat 4).

They're well worth the $65 annual fee for the Visa card.