It really depends on what you mean by improving. It might mean something directly quantifiable like crime rates, or it might be something intangible, like it just being cool and fashionable to live there. The hipsters see the latter and descend like vultures on it. In the process, they do improve the neighbourhood in a quantifiable way - house prices. But they destroy what brought them there in the first place.

And here's the thing: the hipsters don't need to do this. They already live somewhere. Why not stay there and live their hipster lives? That's what makes people angry - they just invade and colonize, destroying communities and driving people out of their homes for fun.

"That improvement is unfair to the people who ran it down"

Assumption 1: The residents are the people who ran down the neighborhood.

Assumption 2: Gentrification improves the neighborhood.

My previous comment addresses the first assumption. Feel free to disagree.

If a neighborhood is just real estate, assumption two is valid. If a neighborhood is a community of people, and gentrification moves a lot of the people, you have not improved the neighborhood, you displaced it.

Once we question these assumptions, perhaps we can address the root problems.