Yes. They are used. Yes, some codes allow them, some do not. And most plumbers, especially many (not all) of those on this forum do not like them because they say they "fail" -- I have read that the real reason many (not those that post here, on this forum) do not like them is because it eliminates a lot of work for the plumber and hits him/her in the pocket. But that is only what I've heard -- so don't shoot the messenger.