Your question is probably too broad. I can tell you about my experience.
I worked on a project that adopted the .NET version (NHibernate). A naive implementation of loading a single row from a single table was almost two orders of magnitude slower than a raw ADO query. After much optimization I believe they got it down to merely one order of magnitude slower.
In java where the start up time is probably less of a factor. The web server loads java and hibernate at server start instead of while a user waits for a desktop app to start.
Personally I really dislike it. It hides implementation details that are necessary to efficiently manage your data. I've found no real world application that could perform acceptably with a vanilla implementation of a data layer that hides database details.
But that may be sour grapes on my part since I was forced to use it and blamed for
not being able to put enough lipstick on the pig.
