There are some spam issues in the comments, so I had to disable comment editing for a short time. It'll be back once the spam bots are gone. FYI... they leave a comment with "great" and then edit in the spam stuff later, to get around the filter.
(Pretty much always used in contexts where you want to emphasize something or express that you're impressed. NOT used for the literal idea of something not being believable - like a story for instance. That is "unglaubwürdig".)