I used Ruby Data Mapper and liked it. Stuff worked, documentation lined up with shipped bits. Then I learned that it had been abandoned in favor of ROM - supposedly a version 2, but actually ROM is a completely different user experience. Worse, the syntax changes so much even in recent years, older ROM projects no longer work, and there is no information about upgrading projects for the new versions.
ROM has very few learning resources, for example: no books. None of the few example projects I could find worked anymore - if they ever did work.
StackOverflow queries to date:
Active Record 164,431
rom-db 73
ROM seems like a failed project. Too bad. Technically, it looks good. Unless significant resources were applied to overcome the issues I mention, however, I would not use ROM.