In a nutshell

"By deciding to join the US-backed Palestinian Authority led by Mahmoud Abbas, Hamas risks turning its back on its role as a resistance movement, without gaining any additional leverage that could help Palestinians free themselves from Israeli occupation and colonial rule.  Indeed, knowingly or not, Hamas may be embarking down the same well-trodden path as Abbas' Fatah faction: committing itself to joining a US-controlled "peace process", over which Palestinians have no say - and have no prospect of emerging with their rights intact. In exchange, Hamas may hope to earn a role alongside Abbas in ruling over the fraction of the Palestinians living under permanent Israeli occupation in the West Bank and Gaza Strip.  Whether Hamas realises it or not, it has effectively entered into a coalition with Israel and Abbas to manage the Occupied Territories, in which Hamas will have much responsibility, but little power." (thanks Joseph)