Crossfit. As someone who's spent too much time in the past mixing up the same boring gym routine, I can attest to the awesomeness of crossfit. *new religion
There are two main problems with it:
1) CrossFit is exercise. A randomized program like it doesn't provide actual training. A strength program or running program or whatever will have better results. Exercise is great for beginners or for people that would otherwise do nothing, but it's not going to get you nearly as good of results as a more specific program.
2) Which leads me into the next problem. While there are definitely CrossFit gyms that have instructors that teach proper form and know enough to help keep the people they're training safe, the quality of instructors varies wildly. All it takes to be a qualified instructor is to take a weekend class: https://training.crossfit.com/level-one
CrossFit's benefits are really only going to be realized by newbies, and newbies are the exact people that lack to ability to determine if they are getting quality instruction.
There's a reason people like Mark Rippetoe and others distanced themselves from CrossFit.