It's not a "mini brain", really.
Neurones form into networks, and those networks can then function and even be programmed to do stuff. But these networks really are not like actual brains from actual living creatures, because the architecture is so different. Effectively, the neurones are just acting like logic-processing units, much like inorganic components of a microchip.
The "human" part of this is also a bit irrelevant - at least in terms of making an organic computer provide an output, because rat or mouse cells will do the job just as well (a rodent-derived organic computer was able to fly a plane on a flight simulator software package years ago IIRC). Human cells might be preferable mostly because they may provide more insight into the understanding of human brains, not because they provide better computing outputs.
www.bbc.com
