When I first saw them I thought they were just for little kids and would not want those "fake fish" but now I think they look cool, I don't have any in my tanks yet, but plan to try them out soon for a little color balance. I think it's far better than breeding fish for a deformity that it is aesthetically pleasing to a human. I have no problem with most genetic modifications, adding color to a fish will have no detrimental effect to the fish in an aquarium. It might even be better than selective breeding (likely inbreeding).
I could see some who want to keep a natural aquarium with natural looking fish, there are a lot of fish that would be off their list including almost all guppies and endlers. I'm not in that boat, but more power to them.
I think its just preference as long as it isn't harming the fish. I have more of a problem with something like the Gold Roseline Barb where a certain % of are blind. While a very cool looking fish, I balked at getting them once I learned that the side effect was blind fish. Hope they can reduce that deformity some day.