If you want a definitive answer, I'm not really sure if they do any good or not! If the type of contamination we're talking about is light dust then they can't do any harm and may even extend the life of your player. But you must use one frequently. Anything heavier and the best they are going to do is move it around. Chances are you'll buy one and forget about it until you have a problem. No good reaching for it at this stage, it's already too late - if your player won't read a CD it's not going to play the cleaner either. Time for a new laser!
If you want to know more about how CD players work look here.
I hope that this has been useful, informative or just plain interesting. As always your comments are invited, as are your faulty CD players!