In the poem "La Belle Dame Sans Merci" by John Keats, is the knight's experience worth the results?

In the simplest terms, no, the knight's experience was not worth the end result. His experience cost him his life. First, what was his experience? The knight met a lovely, and it turns out, literally enchanting, lady in the meadows ("meads") who was slim ("light of foot") with long flowing hair. Nonetheless, she had wild eyes.

They wiled away the time in the meadow while he made her a flower "garland for her head," and bracelets and a belt ("fragrant zone"). Her response was to give looks of love and whisper sweet nothings: "She look’d at me as she did love, / And made sweet moan."

When leaving the meadow, the knight is entranced and captured in her spell. He sets her on his horse and they spend the rest of the day together, she singing "A faery's song", collecting delicious roots, wild honey, and dew drops ("manna dew"). His interpretation of her behavior is that her actions and attitudes mean ""I love thee true."" However, she never says it, this is the meaning of "in language strange": it's not a foreign language, it is nonverbal language.

She takes him to her "elfin grot." A "grot" is a grotto, which is a cave or a cavern. There, she sleeps deeply, and he kisses her closed "wild, wild eyes."She then "lulls him to sleep," and he dreams. He dreams of dead kings, princes and warriors who tell him he is the slave ("thrall) of La Belle Dame sans Merci, meaning "the lady without mercy." Next, he's on the "cold hill's side" in the misery of metaphorical winter with a lily on his brow, the Western culture flower symbol of death.

The poem is set in a frame set on the "cold hill's side" with a flashback to the story of the poem, which can only be understood by working from the inside out, allowing you to see that she has, for some elfin or faery purpose, killed him and discarded his body on the hill surrounding her cave where he has a rhetorical conversation with himself as his disembodied soul encounters his body and wonders aloud how he came to be disembodied. It is in this disembodied rhetorical conversation that the poem frame opens.

So, no, a few flowers, some herbs and wild honey, a country horse ride with a beautiful songstress, four kisses on closed "wild, wild eyes" was not worth death. Some suggest that in this poem Keats is lamneting the deaths of his family or his own upcoming death from tuberculosis.