Save your gonads (and battery) !
Right, ehh, what? Save my gonads?
Yes! You will find more information in this thread, and the other threads referenced in that thread, but it comes down to this: 3G radiation >>> 2G radiation. Using 3G is nice, but the radiation may prove harmful. While I don't know if 3G radiation is actually harmful, one could argue that less radiation can't be worse. Besides, less radiation === less power === less battery usage. So you can save some battery as well. Why use 3G when 2G suffices? And that is exactly what WMLongLife does, use 3G when you need it, and 2G when you don't.
LongLife because it decrases the chances of phone-induced sickness, and the battery lives longer too! Keeping the device in your pocket will fry your gonads eventually if it keeps burning 3G - think of your hypothetical future children!
All theoretically, of course! Also note that if you are a girl (bloody unlikely), you may replace gonads with other organs of your preference.
So what does it do?
Right now, not that much. A short time after your screen goes off, it switches the device to 2G-mode. All background mail checking and such will go through 2G instead of 3G. If you turn the screen on, nothing happens. However, should you start one of the applications from the internal app list (at this time only Opera9 and WMWifiRouter), it will switch your device to use 3G. It will switch your device back to 2G again when the screen goes off again (it waits a bit first, though). Also, it will disconnect your cellular data connection if it has been inactive for about a minute (unless you are running one of the apps from the internal app list).
Obviously it takes into account wether you are making/taking a call or already talking and leaves stuff alone if that is so.
The application will not kick-in 2G mode when you initially start it, until the screen goes off and stays off for a minute!
Just start the exe on your device, dont close it, and go do your thing like you normally do.