I've never changed major plot elements based on reader feedback, but I have beefed up aspects of a story that became more popular than I had originally anticipated. Chewy in Panhead, for example. I had always planned to have a dog in PH, but a few people were really taken with him, so I consciously wrote more of him into the story.
For me, as a relatively new writer, reader feedback helps me improve. When enough people comment favorably on a particular thing, I can see that perhaps I should use that device or technique more often, that it's a successful one in my style of writing.
Plus - and this is purely ego stroking - it makes me feel really good to know that what I wrote, something out of my own head, resonates with people, makes them laugh and cry, makes them want more. The satisfaction I derive from that is unlike anything else I've ever experienced.