It depends upon what other mutations come along with that one. Most likely, no, your chances for reproducing and, thus, passing down the mutated gene would be slim and the mutation would die out with yourself. Even if you did have another mutation that was considered good it would have a slim chance of being passed down because of your unlikelihood to procreate.

That might be a good mutation. Genetics is all around us now.

There are so few cases of a single organism's mutation having any effect on a species as a whole... hammerhead sharks are one of these few. You may think their heads are an adaptation, but it really was one mutant that started it. Adaptation usually involves genetic drift of whole groups within the species, i.e. many, many members would have to share the same mutation for it to last. Aside from special cases, no single person's mutation will affect the gene pool, regardless of how much they reproduce. The "normal" people will completely overwhelm the mutants genetically when considering the whole species. The mutated gene will get diluted into oblivion. But, in the verrry unlikely case that the mutation would be present several generations later, it would probably only count as an adaptation (what you would call "good" mutation) if other beneficial genes hitch a ride with the mutant one... the mutation would have to make it more probable that these beneficial genes would be transmitted successfully (maybe they transmit together as a gene cluster), and those piggybacking genes would be the likely cause of benefit to the species. Hope I make sense...

Yes, if it also gave you protection against diseases that killed many. A mutation that made you mentally retarded but gave 100% protection against malaria would probably flourish in malarial areas. It depends how mentally retarded, of course. -10 IQ poins, +100% malaria resistance = excellent. but if it was -50 IQ points, perhaps no so good.