Lots of comments about not being able to "just turn up and fly." My wife and I are in a boat club that's structured the same way--one flat annual fee and you can reserve a boat any time you want. Only one reservation at a time.
Never once have we been unable to reserve a boat, even on a holiday with only a few hours notice. I assume the same level of availability would be possible in a flying club, dependent of course on the ratio of travelers to planes.