Is "brain training" effective? 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.  

Questions


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.
I believe the hype about brain training got started with Susanne Jaeggi's paper  on n-back training and its alleged improvements to fluid intelligence (Gf) and working memory (WM): 

The experimental design was called into question by papers that followed and tried to replicate similar results. Most of them could not detect a maintained improvement in WM and Gf. Further, most studies did not find it to generalize to other skill domains.

I'm not sure how to interpret the third question.

Numerous studies have shown that mental stimulation is effective in reducing the risk of Alzheimer's disease. The more interesting question is probably whether brain training can ameliorate attention deficit...