Performing negative repetitions can provide enormous growth possibilities provided it is done right.  The biggest problem associated with doing negatives is the fact that people do too much.  1-2 sets maximum is all you need, and sometimes that is even too much.  They are so damaging in fact to muscle cells that it often can double the time it takes for an individual to heal from a particular workout.  With this in mind it's important to take extra care when training in this fashion. 

Why are negatives so damaging anyways?  Well, it all comes down to the way your muscle cells work.  A muscle cell follows what's known as the all or none principle.  Basically what this proclaims is that when a cell contracts it does so with 100% of force.  Your body then simply fires the appropriate amount of cells for the desired movement. 

With this in mind, we now can see what happens when a muscle is subjected to the stress of a negative repetition.  You basically have a cell that is contracting as hard as it can.  When you load weight on the bar that is greater than what the cell can contract against the cell gives.  Simply put, it's torn apart.  This causes tiny tears within the muscle fiber itself which in turn stimulates growth. 

People often wonder why they are so sore after negative training.  The fact that it's actually tearing the muscle apart and causing tiny tears is what causes this deep soreness.  It damages the nervous system along with the individual muscle cells.  You can quickly see how overtraining can rapidly set in - there's only so much a muscle can take before it's torn apart beyond repair. 

So what's a person to do - how much is needed to stimulate maximum muscle growth?  With everyone's metabolisms and body systems different, it's hard to say.  My suggestion is this - start at one and work your way up.  If you've never done negatives before chances are you're going to get sore with minimum reps.  Remember, they key is to stimulate muscle growth, not bombard a muscle into submission.  It defeats the purpose.