We've had quite a few questions about "brain training" on this site (see questions tagged brain-training). And the effectiveness of "brain training" has been touched on in several questions (this question on the definition of brain training, this question about lumosity, and this question about n-back training).
That said, I agree with @Chuck's suggestion that it might be useful to have a canonical sceptics-style question about the effectiveness of brain training.
- What is the effect of completing "brain training"?
- Is there any evidence for domain general benefits to cognitive functioning that extend beyond the specific task practiced?
- Is there anything particularly "neural" or "brain"-like about brain training?
- Is there any evidence that brain training prevents any neurological diseases?
Naturally, it would be good if any answers are backed up by independent scientific empirical research. References to and summaries of recent scientific review articles would also be particularly useful.