I have only heard flouride is bad for children. Now, this is second hand information, but children get flouride in adult toothpaste (childrens toothpaste doesn't have it) and they get it in gum, and tap water. Flouride is bad for developing teeth or something so children are getting messed up teeth (the ones who drink tap water, use adult toothpaste and eat gum all day).
Now for adults I don't think it is a huge problem. My only reasoning is my mom is a dental assistant and great friends with one of the top oral surgeons in Chicago. He has told us a few things, since he is very passionate about his job, yet he has not once mentioned not drinking tap water.
Remember, assuming it is bad the more important question is how bad is it. The difference between a cure and a lethal drug is dose. Its not "Is flouride bad" its "is the dose of flouride that an average human gets drinking tap water bad enough that I should stop using it." I don't mind taking alcohol in small doses, but alcohol can easily kill you so I would probably decide not to get an IV of everclear because studies on rats have shown that massive alcohol intake has killed them. Nicotine is also lethal as is every drug on some scale.