How much can I safely plug into a light socket converted to a standard outlet?
And I am concerned about the poor light socket.  Most standard light sockets say something like


  100W max


This refers to the lamp that you can safely use in that socket. The main variable that goes into that calculation, I believe, is the heat dissipation: most of the energy produced by an incandescent light bulb is in the form of heat, and the lamp fixture+socket has to be able to withstand the heat put out by the bulb.

However, there are other factors, namely, the current rating the wires leading to the light socket and the electrical connections inside the socket itself. If the wires and connections are safe for under 1 Amp, then you can't use a larger than 100W bulb (given 120V AC). In the end you cannot know what part which factors play in the power rating of the socket, and so I would advise to find out the power rating of the light socket itself, and not exceed that either.

Bottom line: the "Handy Hutlet" might allow 660W, but the light socket is unlikely to, and you'd be wise not to exceed the power rating of the weakest link in the chain.