I dunno about crossfit, but I took a les mills body pump class at the gym a few weeks ago and just about barfed. And I run 8 miles a week and lift 4 or 5 days a week I think it's because I actually used an appropriate weight for every exercise and all the old ladies were squatting with 5 lbs on their bar h:

:edit: and on the crossfit vs. whatever else debate, I'm going to say the same thing I've always said. Once you've got a handle on the basics and the form (aka you're not doing it wrong) pretty much any legitimate workout out there will get you into great shape as long as you have the motivation and don't cut corners. If going to a class or a club gives you the accountability to actually do the work, good for you. If a personal trainer works, good for you. If you can do it on your own using videos or a workout you found online, good for you. The point is to stick with it and put in the effort. If you can do that, you're pretty much guaranteed results.