Anyone had experience with the $10-$11 w5100 shields available online? Look like cheap knockoffs -- Do they work?
That depends. Take a look at the pic on this post. Do you see the solder bridges on the w5100 IC? There should be no solder bridges.
http://forum.arduino.cc/index.php?topic=178633.0
I could not find a pic of that type card on Google Images without a solder bridge. 
A0159072:
Do they work?
I have three, they all work fine, so I guess the answer is "yes".