Paul Ricœur famously suggested,"phenomenology in the broad sense is the sum of Husserl's work plus the heresies stemming from Husserl". If this is correct, at least at first sight, there is an obvious two-way strategy to measure the full extent of phenomenology's contribution to philosophy.
On the one hand, each phenomenological "heresy" should be measured against the backdrop of a thorough understanding of Husserl's published and unpublished work; on the other hand, Husserl's specific variety of phenomenology should, in turn, be contrasted with the conceptual transformations and innovations of its "heretic" offspring, such as Heidegger, Reinach, Fink, Langrebe, Ingarden, Patočka, Sartre or Merleau-Ponty-to name only a few.