I'd like to know its precise meaning. Does it mean "Tell me what you think about it (the next time we meet)"? I don't know how to use this phrase.
- Ce collier, je l’ai fait moi-même. Vous m'en direz des nouvelles !
This expression often implies that I believe that you'll like it very much - whatever is being referred to (and thus we'll talk about it again). See for example the Wiktionary in French.