Glad you added this link for discussion. I've been curious about crossfit for sometime and I'd love to see HN comments about it. I like that it kind of challenges conventional wisdom.
That said, there is a somewhat off-putting Crossfitter ethos of overdoing it and injuring yourself, and some of the folks give off a cultish vibe. But if you're smart about learning the movements and knowing your body, I really think it's worth trying. I was skeptical and now I'm hooked.
Sustainably working out hard three or more times a week and benefiting from it requires somewhat rare genetics or growth hormone and steroid injections.
You have to listen to your body of course. When I first started, sometimes I would take extra days off when I knew I needed recovery time. As with any activity, you have to be smart about what you're doing.