As I have worked to shape a definition-- even a starting place for a definition-- of "success" in church planting, I have become increasingly frustrated as I have realized that every component we develop by which to measure "success" is either a complicated re-statement of the original question, or requires a myriad of its' own qualifiers. Furthermore, I have been plagued by this question: "Is x an essential component for success, or a natural by-product of it?"
Rob has proposed a preliminary definition of success (which he takes from Marcus Buckingham's book, The One Thing You Need to Know): "maximum impact sustained over the longest period of time". Granted, Buckingham is a social scientist and business consultant (an orientation of "success" that many of us desire to distance ourselves from), but I think this can be a helpful starting place nonetheless.
Clearly, "success" has often been structured around numerical growth as opposed to the cultivation of authentic biblical spirituality and the reproduction of reproducing disciples; and defining "success" in terms of maximum impact could clearly lead one down that road. However, no matter how far we want to distance ourselves from the "church growth movement", we must face the reality that "living things grow" (Howard Snyder's chapter in Evaluating the Church Growth Movement has been extremely helpful for me with respect to this tension).
So, if we choose to use maximum impact as a working framework, we are left with the question, "By what measure do we quantify maximum impact?" (This is, of course, the success question restated in new terms. I have officially made zero progress).