I've never used one and not heard anything good about them. If I need to clean a really dirty chain I take it off and rinse it with parafine.Ive always used rags, brushes and a bit of petroleum and it has always worked.
But you see these things in the LBS and wonder if they will make life a bit easier.
There is a lot of satifaction in getting a bike chain, cleaned and oiled.
I need to get a life.
You'll buy one anyway. Use it...realise it's crap and put it in the shed to gather dust. A rag is all you need.