This is a nifty feature that has snuck up on some of us. Thanks for continuing to develop and evolve discourse! 
I’m not sure yet how I feel about the feature, but my inclination is to let it run on my site for a while to see what happens. It seems interesting, but it may feel a bit random to the person who receives it - especially if (as in the case of the first badge awarded on my site) nobody even replied to their post that triggered the award. They might be wondering… if my post is so great, how come nobody replied to it? 
Maybe you could also require that posts have at least one reply before the badge is awarded?
Also, and this is obviously an audacious idea, but could the functionality to send badge-triggered PMs be exposed somehow in the admin UI? It would be interesting to be able to send more of these types of emails but also to control it ourselves as admins. I can imagine some site will be happy with displaying the badge but not want to send the email, or vice versa.