Any first sighting is going to be life-changing in some way. Either it will affirm one's belief, confirm existence for the open-minded, or fundamentally contradict with a skeptic's world view (or at least cause a skeptic to question himself - I, myself, refused to accept that the individual I had encountered was not a man until my second encounter).
After that (the "OMG there one is" moment), there is the intensity level of the encounter. Was it a sighting at a distance? Was the sighting close enough to allow interaction? did the bigfoot behave in a evasive, passive, benign, defensive, intimidating, threatening, or aggressive manner? Did the witness feel that they or someone with them was in jeopardy? Did the witness, for example, feel the need to protect a family member if the bigfoot made a move, possibly with the realization that it might be at the cost of their own life? Or worse, did the witness feel helpless to protect a child or spouse?
Given the long history of legends and lore regarding bigfoot-like creatures around the world, one must also consider that over the millennia the competition between us and that class of hominids may be ingrained in us at the visceral level as well, as it is with most categories of animals that have preyed on or threatened our species throughout time. We may be hard-wired to respond at the gut instinct level to an encounter with them. I even think that this manifests as the motivation behind the behavior of the most adamant of skeptics.
All of these things are potential factors influencing the psychological impact of a sighting or an encounter.
One thing is for sure, a sighting or encounter is not likely to be a "so what" moment.