I will preface this by saying CRISPR has a long way to go and a lot of bugs need to be worked out, and despite us having sequenced the whole human genome there's still a lot we don't understand about how both genes and environmental factors play into disease. Autism is a prime example, with more than 60 genes identified as being somehow involved in autism and a number of maternal risk factors that all seem to merge into the nexus that results in an autism diagnosis. Alzheimer's and most cancers present this way as well. There are currently just too many unknowns and it's likely to take decades more research before we better understand the genetics behind them.
The question is do we want to?
I don't think the question is do we want to but "how far do we go" and "for what purpose?"
You did mention sickle cell anemia and there are many others that can be boiled down to a single faulty gene such as Huntington's, Tay Sachs, and cystic fibrosis. I don't think anyone short of the disgustingly religious will say we need more babies with Tay Sachs in the world (because it's part of God's plan, of course). I think the majority will agree that if we can prevent people from developing a disease that will end their life or cause them immense suffering, we should do it.
But lines get blurry really quickly as to what constitutes "life-threatening" or "immense suffering." With pre-implantation diagnosis people have already flirted with the ethics to the point of absurdity. Many disability advocates have tried for so long to convince everyone that there's no such thing as being disabled, there's just "differently abled." There have been cases were two parents with dwarfism wanted to use PID to produce a child with short stature and two deaf parents wanted to insure they would have a deaf child. Isn't that what most parents seek? A child who is "like them?" While I don't support eugenics, it's hard for me to argue that we should deliberately create
children with what society at large would consider a disability simply to soothe a parent's vanity.
It gets even crazier when you start toying with things like homosexuality, which has a strong genetic and fetal environment role. Should parents be able to select for or against homosexuality in a child? I can just imagine a bunch of gay-hating bigots in a fertility clinic wanting the "gay gene" cut out of their child.
Ultimately I think this is no different than any scientific breakthrough: with great power comes great responsibility. GMOs had the chance to shape the environment and revolutionize agriculture but they've been badly mishandled. I could see this going the same way, and it would be nice if there were laws put in place with the advice of scientists, geneticists, and physicians to say which conditions CRISPR should be used for. Absolutely use to it prevent neurofibromatosis! Don't use to it prevent a child from being able to see just because it has blind parents.