I think the fears of GMO mosquitoes somehow impacting humans directly is ridiculous, but I am skeptical about potential unintended environmental consequences. It's impossible to know what will happen to the mosquito population once you introduce an unknown factor into it.
Had a conversation with a friend earlier this month about logical fallacies in the media. One that comes up a lot for genetic modification / agriculture / ecology is paralysis by analysis. In these contexts, the arguments usually fit the definition of this fallacy, but it's also one of the most basic tenets of ecology that we don't know what large changes to a system will do to it. It's rather frustrating, but who knows. This'll probably end up being tested, since the stakes are fairly high. Only then will we finally see the consequences, intended or not.