How reliable are stick shakers? I've read many accident reports, and seen reconstructions, where instrument failures, disagreeing sensor sources and auto pilot faults have led pilots into believing that they are over speeding when they are not (think AF447), are descending when they are actually climbing etc.

It seems that in all cases where this has led to a stall, the stick shaker has activated but the PF does not take the correct immediate action for stalls, instead continuing to believe the erroneous information they are being fed.

I understand the psychology of why this might be but is does strike me that many lives would have been saved if power up nose down had been applied.  I've not found a case yet where the shaker was wrong. This leads me to wonder...

How reliable are stick shakers?  Should they always be trusted?

Some older types even had an automatic stick pusher but this does not seem to be present in modern designs.  Is this a consequence of shakers not being infallible?
The Australian Transport Safety Bureau recently published a comprehensive research paper on reported stall warnings in high capacity aircraft - http://www.atsb.gov.au/publications/2012/ar-2012-172.aspx 

The report says that 30% of all stall warnings were false. Note that stall warnings aren't limited to the stick shaker.

The report says that of all reported stick shaker events, 16% were false warnings. Have a read of it, you'll find lots of interesting bits of information.