I didn't know it existed up till now, but i've read up on a few Dutch forums about it
they are all very positive about the program, its cheap, its fun, you will probably learn much more about the country from these locals than you would in a hotel
it does appear some hosts require you to have the membership card, but overall the rules are not very strict, sometimes you get a nice apartment on your own with great breakfast, sometimes its just a childrens bedroom next to the hosts room, sometimes you share the table with other cyclists, you never really know
most people seem to call just 1 or 2 days up front, sometimes the same day is possible too; and if they don't have a room there are still plenty of youth hostels etc. as an alternative
seems like a good idea to me