The "Clarion chain" is actually Choice and their FF program is Choice Privileges. The chain also includes Comfort Inn, Quality Inn, and Sleep Inn.

They actually have props all over Europe, but the quality/desirability varies, so best to check with tripadvisor or the like before booking.

They also run promos in which it is easy (with some hotel-hopping) to rack up an enormous number of points, which can be spent on free nights or converted to miles.

All in all, for me it is a worthwhile secondary chain to Hilton/Marriott. I've only been a member since March, but I scrounged enough points by June to book a couple of nights in a 5* property in London for next month.