We've had them here in Belgium as well for quite some time. (Not that surprising, we can share translations :P).

Personally I'm not a fan of it. Even in transit, I often have a backpack with me and can read the paperback books I have just fine.

Plus, what's a better replacement to me, is just having the kindle app installed on your phone. When I don't have a paperback with me, I just open the kindle app and read from there.

Which arguable is even easier to carry for being in-transit than a small book, since people usually have their phones weith them.

The format was invented by someone at Jongbloed publishers and printers, which specializes in printing on thin paper, usually used for bibles and hymn books.

I've nevet seen a hymnal with thin paper. They are generally rather rugged because they are designed to hold up to long term use by multiple people.

Is it common where you are to print them with thin paper?

Most of those used to be combined bible/hymnals, but the most recent (2013) hymnal of the mainstream protestant churches is so large that it is also printed on thin paper (maybe not the thinnest?).

Those combined bible/hymnals are usually personal, not for multiple people, and they're most common among denominations that only use the 150 psalms and a small number of hymns on texts directly from the bible, although I do have one with the bible and the entire Liedboek voor de Kerken (from 1973). Normal hymnals usually used to be printed on normal paper here.

And they were available in the US about 5 years ago as "flipback books". I have John le Carré's "Tinker, Tailor, Soldier, Spy" in the format. It is kind of cute and smaller than modern phones, but the claims that were made at the time that it was going to be an ebook killer never materialized and eventually they stopped selling them here.