It is not harmful and can be a big help in beating the habit, especially used together. Nicotine patches and gum perpetuate the problem. It's like giving methadone to a heroin addict, one addiction for another. Some side-effects include increased appetite, rapid heart rate, irritability and muscle pain. Smokers also need to include behavior modification through hypnosis or other programs.
Physical addiction is part of the problem, but psychological addiction is just as important.
Here's a site than can help. It's called No Smokes .