I have always thought that serrations on a knife made it look overly aggressive, but having recently invested in a Spyderco Sharpmaker (and read the information on their website), I can see that there is a reason behind them. The curve offers the ability to create a longer cutting edge than the length of the blade - if it is sharpened in a series of curves, the edge will be longer. Also, the points created by the serration help to protect the concave cutting edges meaning that the blade will stay sharper for longer. The points assist in piercing the material being cut and the curved edge perpetuates the cut until the motion is stopped by the "cutee".
Even though I now understand the reasons, I still do not like to see a knife with serrations, and cannot understand why a blade would need to have both plain and serrated portions along the same edge. I can see that there is a need for "Rescuse" situations, where seat belts etc need to be cut, but for the general public, there is little need for such aggressive looking tools.
ATB
Ogri the trog