There is an article on BT101 called "Taking a Shower in the woods" that you might be interested in. In additional to some discussion about various showering options it also has quotes from additional cyclists who use these kind of devices.
I have two of them myself. Currently I am leaning towards using my water bladder with the optional shower attachment.
If the sun isn't available or time is of the essence then an alternative is to carefully heat some water in a stove, mix it with some cool water and fill up the shower bag. Of course this assumes that you have a stove with you.
http://www.bicycletouring101.com/CampShower.htm
~Jamie N
www.bicycletouring101.com