I wrote an article about using Home plug ethernet powerline adapters in the past, but I mainly discussed using them for connecting a laptop directly to ethernet for a faster connection to a NAS which houses my digital photos.
Since I liked the way the plugs increased speed for the laptop, I decided to put them in a couple more spaces. I started with one next to my usual position on the couch, but I also wanted one next to my favorite chair and headphone setup:
I own both of the models pictured below.
Now I have three wired Airport Express units and all three are expanding my wifi connection. It helps because I have three additional Airplay receivers (Phillips speaker, AppleTV, and Airport Express) that are still wireless getting a stronger wifi signal now.
Another benefit is that when I listen to music on these wired Airplay setups, I can send the audio signal to them without it ever leaving a wired connection for wireless, if I prefer. I use a mac mini for serving my iTunes library, so I can control what is playing via my iPhone's "Remote" app and select where the signal is playing. The only thing wireless is the remote. I can also use an app like Rdio on the mac mini and control it via my iPhone. While direct streaming does have benefits, like when you are running a microwave nearby, having wired Airport Express units expanding the wireless network via wired connections, makes the wireless stronger everywhere.
As you can see, these power line ethernet adapters, working in combination with Airport Express units, can be great for expanding your wifi network and for optimizing your audio. For me, they were money well spent.