The fundamentals of Crossfit are entirely sound. A warmup followed by some subset of bodyweight exercises, calisthenics, rowing, running, climbing, and (olympic) weightlifting is a completely reasonable fitness regimen. Being sensible is a matter of leveling the workout to your own particular circumstances: age and overall fitness being the two biggest factors.

But yeah I've also seen the videos on youtube of people doing heavy barbell back squats on a bosu ball and similar, to use the technical term, stupid shit that will get you hurt.

Mark Rippetoe has one of the better criticisms of CrossFit: it's just an exercise program, not a training program. Proof is that the athletes that compete in and win the Crossfit games don't actually do Crossfit to prepare, they have specific training regimens for those games.