My LAN and WLAN is always turned on so my Apple Airport Extreme is running 24/7 and it is a very good at that. The WLAN covers most of my house but there are some places where the signal is low so I have often needed some sort of a wireless network repeater.
I am not a big fan of WLAN because a WLAN is less secure than a LAN. But the WLAN is extremely nice to have and when you can sit in the garden with your MacBook and access your NAS it makes sense.
The Apple Airport Express device is a easy way to extend the current WLAN and even provide a new feature: AirPlay. With AirPlay you can connect to iTunes on your MacBook and play music anywhere in the house that has some sort music device.
I find Airport Express very useful as the device can be placed almost anywhere it just needs some power in order to function. The Airport Express can not act as a repeater unless the current network is an Airport network. I was a bit annoyed when I heard that.
I definitely recommend the Apple Airport Express if you want to extend your current WLAN.