Hello,
I used the F&F voucher at the HI Mulhouse in France location. Rate was only 50 euros per night and I was upgraded to a executive corner room, with balcony, views on the Vosges. They gave me also 50% discount on breakfast. They thought I was also working in a hotel of 6C... they did not ask what hotel it was
Never thought this was such a helpful site. Thanks!