By definition, gentrification is the process of changing an area from low-income standards to middle-class standards. To state whether it is good or bad depends on one's perspective.
Low-income neighborhoods tend to be blighted and crime-ridden. This is not because the people themselves are "bad," but because their ability to maintain and protect themselves is compromised by the very factors that contribute to their economic status. They are generally people with limited ability to raise themselves above the poverty level. This may be due to a variety of reasons, including lower levels of education or health issues.
As far as the economy is concerned, gentrification is a good thing. It changes an area of low value to one of higher value. There is a raising of the tax base. There is the introduction of new business. The higher class of citizens demand, and provide for, more aesthetically pleasing surroundings. Typically, the standards of living are increased.
However, what becomes of those occupants who cannot, or have no desire to, keep up? They become displaced. If they are lucky, they find alternate low-income neighborhoods to inhabit. If they are not so lucky, they become part of the growing number of homeless.
In order for gentrification to become a win/win situation, the new citizens of the neighborhood MUST be responsible for aiding those that they have displaced. After all, they have profited by being able to "buy low" and have the opportunity to restore these blighted communities. Their ability to do so is at the expense of the people who have served as place keepers until someone more affluent could take over.