I'm sure that this things do work - but like you I'm a little skeptical too.
There is no substitute to regular cleaning with a limescale remover. The little and often approach works best.
Another approach is to 'remove' the limescale by softening the water. This is a little more involved, but have other benefits such as improving the taste of the water. This is a softener that we have used in the past, as it is a DIY fit, cheap and comes with a lifetime warranty: http://www.eddy.uk.com/diy_home.htm
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!