Let's assume that at some point in the future we'll be able to build computer systems capable of behaving just like a human brain. There are three obvious ways to get there. One way is to build a system that emulates an actual human brain, bootstrapped by copying the brain of an existing person. Another way is to emulate a human brain, but bootstrap via a learning process as children do. The third way is to build a system that doesn't work like a human brain but reimplements the functionality in whatever way seems most suitable to the hardware we have built. I believe that copying the brain of an existing person is by far the best path; the other options are much greater threats to the survival of humanity, and indeed intelligent life.
Fully reimplemented intelligence would be very hard to predict and control. Reliably engineering for virtue will be extremely difficult. In its early stages the system would likely be narrowly focused on specific goals (perhaps military or corporate), and there is great potential for catastrophic bugs, such as runaway goal seeking that accidentally devastates or exterminates organic humanity. Even if you think a complete transition from organic to machine intelligence would be a good thing, we could easily by accident end up at a dead end for intelligent life, for example if humans die off before all aspects of machine self-reproduction are automated.
On the other hand, if we copy an existing brain, we will know roughly what we're going to get: a disembodied human mind in a machine. We can even choose particularly virtuous people to copy. The process will no doubt be lossy, but people with severe disabilities cope with sensory and motor deprivation without going mad, and our emulated minds probably will too. We can be confident that a benevolent, thoughtful person who cares for the welfare of humanity will still do so after transcription.
The emulation-from-infancy approach falls somewhere in between in terms of risk. The result would probably be more like a human than fully reimplemented intelligence, but it's hard to predict what kind of person you would get, partly because they would grow up in an environment very unlike what we would consider a favourable environment for children.
I'm optimistic that an emulation approach is more likely to succeed technically than a full reimplementation approach at producing a self-aware general-purpose intelligence. Futurists tend to imagine that mere aggregation of computing power will bring on intelligence, but I think they're quite wrong. A great truth of computer science is that hardware scales but software remains hard. Porting existing software is expedient. However, copying an adult brain may be a lot harder than bootstrapping from infancy.
I'm not sure that any of these things will happen in my lifetime, or ever. There is great potential for God or man to frustrate our technological advancement.