These are so friggin' cool...I've never sat in one but have heard that they are crazy comfy. They'd definitely add some eclectic flair to anyone's outdoor (or indoor) space. I've never seen them on CL so I'm not toally sure why they are so much cheaper than retail - but it's not really my job to answer those questions, now is it...?
Here's a great article on the Acapulco chair's rise to power from Core77.
Ocho Workshop makes them in all colors, and even leather (!), for adults and children.