Insufficient copies will be probably always detrimenta, if that means bad 😆
Extra copies could be beneficial, if there where some good genes*), but usually that leads to various genetic disorders, like the Down syndrome. Look at these links
http://en.wikipedia.org/wiki/Genetic_disorder
http://en.wikipedia.org/wiki/List_of_genetic_disorders
😉

EDIT
*) and vice versa, deletion could be beneficial, if bad genes were deleted 😉

It would all be detrimental, since you stated whole chromosomes. It all has to do with dosage compensation of gene products. This is why even the double X chromosomes in females has to have one of the chromosomes be inactivated, so as not to cause detrimental effects to the organism. While the only X in the male (along with the Y chromosome) is fully activated.
Balance is the key. You can not have too little of one gene product and too much of a gene product.

@kolean
not necessarily. Yes in humans that is the case. But for example in plants poliploidy is often seemed and it has been a beneficial thing for plants in cold climates. And if you have an organism like an ophioglossum fern which has its genome divided between around n=600 chromosomes, an extra chromosome has so few genes that it may just be beneficial in some environments.

True, I do seem to assume animal as the organism in question.

Yeah, people usually assume animals as all organisms…
But in this case, as is it in Human Biology, you were probably right:)