I've been using this with my clients the last few weeks, and while it's definitely polished enough for many uses and one of the better admin tools I've seen, note some drawbacks of this tool:

1) It does not appear to have good support for custom Postgres data types (which is reasonable, given that Motor admin appears to work with multiple SQL databases, rather than just Postgres)

2) Its handling of hiding columns is somewhat hacky - if you hide columns from being shown, they're hidden from inserting and editing as well. This can cause issues if you'd like to hide some NOT NULL columns from being displayed.

3) It seems to inject its own 'ID' column into all 'Resources' in the admin panel, and will not be happy if you happen to have an 'ID' column. (As far as I can tell, it seems that the 'Resources' of Motor admin are precisely Ruby on Rails Resources, and that Motor admin generates Rails classes and Resources for you based on your database schema)

4) It does not allow 'unusual' table names such as names containing ':', '--', etc (This seems to cause the generated Ruby class names to become invalid)

To cope with the above issues, I've created Postgres tables for the express purpose of being managed by Motor admin, with the admin tables being sync-ed with the backing tables via triggers. I've attempted to use views instead, but it seems that Motor admin does not recognise views.