I always preface comments on terminal ballistics and calibers by stating that I am not an expert on the subject, but I have done research and have a basic understanding of physics. While some may scoff at the seemingly "small" increase in velocity of the .357 sig over a +P 9mm, 150-200 FPS is not trivial in a comparable 124/125 gr bullet, with a substantial increase in overall energy. In a well constructed JHP, the effects can be pretty substantial in terms of permanent stretch cavity length. It is probably why you see so many federal/state LE agencies using this round as it does come somewhat close to .357 mag specs (which had terrific street credibility). And when maximum performance counts more than cost, there is something to be said about the potential longevity of this round for LE use. So echoing what others have said, as long as LE uses it, it probably won't be going anywhere. That said, the expense in shooting, less capacity, reload difficulty, additional wear on guns, etc. will probably keep this from ever becoming as popular as the big "three" semi-auto loads (9mm,.40, .45).
TNOutdoors9 does some nice, simple, and quick SIM_test ballistic gel testing.