Is it possible to print humans?
The easiest answer to this question is NO. We will not be able to print humans any time soon, if ever. Despite the potential of the technology, it will likely still make more sense to use stem cells to fight genetic diseases, create limited cellular masses such as hearts and other organs, and to do reconstructive surgeries such as skeletal repairs.

The reasons vary but will include:


The human being is not just a single series of cells, we are a complex melange of integrated systems connected together in a predetermined manner by our genetic code.
The standard 3D printing process might be most effective for producing a single type of cells or a simple organ whose structure is simple and whose configuration could be managed with computer aided design to produce the cells needed at the time of the organ's creation. Even an organ such as a liver will need connective tissues, for blood to be provided for the organ and those cells would need to be integrated as well.
Brain tissue would be the most difficult part of this process. Even having the tissue matrix derived from stem cells would not give us the understanding of how to create an actual brain from a printing process, which uses a layering method to deliver the material into a 3D structure. 


It is certain that some things we may do in the future may be done by printing output that resembles cellular matter, i.e. bone repair and restructuring.


In the case of fixed materials such as bone, a 3D model could be taken and then replicated on a printer. The replicated material is then integrated into an area of the body and allowed to either grow in place or artificially stimulated to grow to replace the damaged area. 
I use bones as an example since they have very limited purposes and once replicated artificially, would be relatively easy to put in place. This would certainly transform the dental industry, perhaps hip replacement surgeries. 
If the technology could be applied internally to damaged tissues, perhaps spinal injuries could be repaired and with proper rehabilitation, a person who may have lost mobility due to an injury could have nervous tissue regrown and learn to walk again. 


The potential for specific cellular development is outstanding but the method for growing complete humans has had millions of years evolution and not likely to be surpassed by Humanity in the next thirty to one hundred years unless we come to a far greater understanding of the human genome and the genetic/cellular relationships between systems in the human body as a collective whole.