Everyone makes their own choices, but unless there is a good medical reason for introducing cereal early, I would not do it. The scientific evidence shows that introducing cereal early can cause diabetes and some postulate that in the bottle it can increase the risk of obesity. I am sure the truth is somewhere in the middle, but I wouldn't do it unless absolutely necessary.
See:
"Throughout most of human history children were exclusively breast fed for the early months. During the previous generation or two, when bottle-feeding became very popular, rice cereal was often put into the bottle at a very early age. What were the results?
Most children seemed to thrive. A small number of children, though, did not tolerate the addition, because their sucking and swallowing actions were not yet fully coordinated. They inhaled small amounts of the rice cereal into their lungs, which led to pulmonary problems.
I’m much more concerned about a subtler issue. Babies are born with a wonderful mechanism for knowing how much food they need. During the early months, they take their cues from the volume of what they drink. Adding cereal derails this mechanism. It forces them to take in deceptively large amounts of calories. It teaches them to overeat.
By starting with a spoon, resting between bites, and stopping when your child lets you know he’s full, you will be laying an excellent foundation for good eating habits throughout his life.
A major study looking for the causes of obesity found that short-circuiting young children’s self-regulation of how much they eat is a major cause of later obesity.1 Cereal in the bottle does just that.
Babies that are fed this way may appear to be unaffected – but those few weeks of added convenience may result in a lifetime of struggles with weight. This common practice may have contributed to our being the most obese generation in history."
http://www.drgreene.com/21_861.html
and
"Two new studies in the latest Journal of the American Medical Association show a link between diabetes risk and a baby's age when cereal is introduced into the diet."
http://www.cbsnews.com/stories/2003/09/30/earlyshow/contributors/emilysenay/main575958.shtml
I say this even though I did introduce rice cereal briefly for reflux at 4 months (until I switched to a non-nutritive thickener).