to me, an appeaser is someone who agrees to a compromise that only his opponent benefits from, with the goal to avoid further conflict.
Appeasement is usually one-sided, whereas a classical compromise will require both sides to give up something, so that an agreement can be reached.
Usually, appeasement will result in the appeaser to be perceived as weak and is therefore not likely to avoid further conflict, unless, of course, the other party got what it wanted, and is satisfied. I dare say, however, that in most cases, appeasement was not successful in finally bringing a conflict or argument to an end. Why do I say that ? Because I believe that successful negotiations require that both parties respect each other and treat each other as equals, and with appeasement, that just isn't the case.
With that being said, the concept of appeasement, negotiations, etc. only applies to coherent entities such as nations, governments, corporations, or individuals. You cannot negotiate with terrorists, because there is usually no one to negotiate with, and their views are usually so fanatical and their demands so outrageous, that a reasonable compromise can not be reached anyway.
Terrorist organizations like Al'Quaida do not have actual political goals anyway. If that were the case, there would at least be some basis for negotiations.
Appeasement is a form of compromise. As with any compromise, the important question is what you're willing to give up to reach a solution.
Thus, there may be situations when appeasement as a strategy could work out.
