Today everyone who came to the Van Gogh museum by mid morning was turned away if they did not have a timed entry reservation. They just kept saying they were full and a timed reservation must be made online. There were a lot of unhappy people with I Amsterdam Cards and Museumkaarts who couldn't enter. Ricks directions seem to suggest that it easier to avoid the line with a timed entry but he doesn't say that is it a necessity, like it was today.
We were there without a timed entry so we got out our phone (Edit: went to a wifi access place) and made a reservation for a timed entry tomorrow ( there were some available times the following day). I think the language in the guide book needs to let people know that without a timed reservation, made online, you may not be able to get in even with a prepurchased museum card.