A good case study builds trust between you and your customers. It’s real, a tangible example of how a product or service you make has helped a real person. It can be very persuasive. They take a bit more effort to write but well worth it.
Within an email, a case study will be more like an overview as it should not be more than two or three paragraphs long. This can be a precis of a longer piece that you have on your website and you can link to that story with a “read more…” link.
Finding the stories is probably the hardest part. Talk to your regular customers or run a competition on your website to find good examples. It is then best to interview the person that used your product or service. This will give you the ammunition you need to write your case study. Ask a lot of questions – what, why, where and when. Find out what was their need. What drove them to pick your company? How was your product used, what was the impact, were there any issues? Did it save them money, make them more productive, more beautiful or more confident? It is the human story that is interesting. Record the interview on your phone, take some pictures and then write it up.