I'm often asked by customers for case studies of successful agile adoptions or agile projects in general. This is definitely a valid request, and yes, such case studies exist. But I'm often concerned that the people making these requests don't appreciate the implications of what they're asking for. My concerns with case studies are:

1. The juicy information is rarely included. The information that you really want to find out, such as what went wrong and why it went wrong, is rarely discussed. If problems, oops I mean "challenges", are discussed at all they're typically glossed over in favor of focusing in on the positives. Although many people want to write up the juicy bits this information is invariably edited out through the company's vetting process. In short, my advice is to take case studies with a grain of salt.
2. Some case studies are more fiction than fact. Although this isn't a problem with IBM case studies due to the governance efforts of my good friends in IBM's legal department (we love you folks, really) it can be an issue with some case studies.
3. The case study may no longer be true today. Stuff happens. Perhaps the case study was mostly true at the time it was written, but now that time has passed problems have appeared that weren't apparent earlier, thus the effort wasn't as nearly as successful as it was written up. For example, a few years ago I ran into the manager of a team that I had read about in one case study, only to find out that once the study was published the key team members left the company to become consultants in that subject area. Having lost these people, who were all very highly skilled, his system proved to be unmaintainable by the rest of his staff who weren't so highly skilled and had to be rewritten. Over time the success story turned into an abject failure.
4. Waiting for case studies puts you in the position of follower. For every case study that gets written, dozens, if not hundreds of similar efforts didn't get written up. Writing case studies is hard, takes time, and the writer seldom gets much benefit from doing so. The lag time between the project completing and the case study being published can be many, many months and sometimes years. The implication is that by the time you wait for several case studies that are similar to your situation you've pretty much lost all opportunity for competitive advantage and are now merely trying to catch up to the organizations who are clearly ahead of you (the writers of the case studies).
5. What has the requester given back to the community? I often hear people lament that there isn't enough case studies, or isn't something close enough to their situation. Yet, when I ask them how many case studies they've written and the answer is usually none. If you want to get you also need to give. ;-)

So, next time you think you need a case study before making a decision, recognize that you may be paying a fairly high opportunity cost for information that is questionable at best.
Further reading:

• Where's the proof that agile methods work?
• Rational's Agile Home Page