In review, using the antenna boosts us from a barely useable connection to a pretty strong connection, but it's not perfect; I found it has to be mounted outside to get the best reception. I'm guessing this is because our windows have a metallic coating. With this set up, we have to be plugged in and can't move around the apartment, but we're okay with that. And now we don't have to give $720 a year to any of the big telecomm companies.
Side note: The USB wifi device that I bought has drivers for Linux, but I found it easier to use NDISwrapper to install the Windows drivers.