What impact did the path chosen by the speaker in "The Road Not Taken" make on his life?

"The Road Not Taken" by Robert Frost is an ambiguous poem that can be interpreted in several different ways. Depending on one's approach to the poem, the impact of choosing the "road less traveled by" can be positive, negative, or negligible. 

One common way of interpreting the poem is as a celebration of individualism and non-conformity. Frost wrote the poem in 1915, and since then many readers have felt that the poem encourages them to strike out off the beaten path and choose a way that suits them, not the way that most people are taking. With such an interpretation, the "sigh" of the speaker "ages and ages hence" is a sigh of contentment for a fulfilled life, and that fulfillment is interpreted to be what the speaker calls "all the difference." That is, the speaker sighs in contentment because the choice to follow his own path leads to fulfillment--more than would have been realized if the more traveled road had been chosen. Thus the impact of the choice in this case is positive.

Another way, equally valid, of interpreting the poem is to understand the poem as being about how irreversible decisions are. With this approach, readers focus on the fact that the speaker "doubted if I should ever come back." Once a decision is reached, it is momentous, and there's no going back. In this interpretation, one could see how the "sigh" of the speaker could be one of regret. In his old age, the speaker may wish he had pursued a different path in life, but that one significant decision in his youth set him on a direction that could not be undone. In this case, the impact of the road chosen is negative.

Finally, the third interpretation, and actually Frost's intent, is that the poem is a tongue-in-cheek jab at indecisive people who make too much of any single decision. Some people deliberate too long over the minutest choices, believing each one to be a life-altering action. In this case, the poem seeks to point out, through melodramatic irony ("Oh, I kept the first for another day!"), that a small decision is really just that--not something that one will reminisce about "ages and ages hence." Or if the person does attribute such undeserved importance to a single choice, it will be because the person tends toward hyperbole. In this case, "all the difference" is to be taken as verbal irony, meaning the opposite. In fact, the decision is not consequential, and the impact on the speaker's life is negligible. 

Frost admitted this poem was "tricky," and readers have taken from it many different meanings. The poem allows for the impact of the choice of the path to be good, bad, or indifferent.