Hi Norman, thanks for the welcome. I would definitely consider myself out of shape, and not an expert on Crossfit. But, judging from what I've seen and experienced, yeah it's possible for people that are at the right level of fitness to tackle L8D3. Here are a bunch of exercises from the Crossfit page. If you scroll down to "the girls" or "wod" sections, you'll see some hardcore workouts.
I think that one benefit of the Crossfit method is that anyone can do the workouts- they just scale them up or down depending on skill level. (Similar to how Simplefit has levels).
Another benefit is the randomized nature of the workouts. It keeps the body constantly getting different stimulus.
So getting back to "why I'm here" - Simplefit seems to give me a good foundation to get back into shape. And I don't need a bar bell, or rowing machine (yet). Plus I'm pretty bad at scaling workouts myself, so I would often end up hurt and not able to exercise. With Simplefit, I'm starting at the bottom. If I were to do it right, I'd join a Crossfit affiliate to get some hands on training, but the logistics don't work out where I am.
Anyone else want to chime in about Xfit, please feel free to add to or correct anything I've said.