I think the need for action to prevent a routing of the rebels/population of the East is important at a humanitarian level. But considering the location of Gaddafi's forces does this new alliance have assets in place to prevent the fall of Benghazi? Libya's surprise announcement of a ceasefire and a halt to operations suggests that the resolution has succeeded in warning Gaddafi off risking such a move.
I think that the decision does has legitimacy. The Arab League sponsored Lebanon's tabling of the motion and at the Security Council it has received 10 votes in favour with none against.
Arab support and the fact that Arab aircraft will surely play a part in the operations mean that the decision cannot be seen as the West imposing its will on the Middle East. In addition the fact that China and Russia choose not to veto (considering their traditional oil/weapons concerns) is evidence of the regional nature of support for placing a break on Gaddafi's counterattack in what has clearly become a civil war. I don't think that oil/weapon sales is the reason for the Western powers supporting the decision. I thought it was very interesting that the Americans played a supporting rather than leading role at the United Nations (this may have also impacted the Chinese, Russian veto decision), I was equally disappointed that the Germans shattered any sense of European unity by abstaining.
So Prime Minister Cameron is correct when he says that UNSCR 1973 reflects 1) demonstrable need 2) regional support 3) strong legal basis.
At a tactical level the superior technology (Typhoons and Mirages against aging MiGs and perhaps one old Mirage) should ensure that the gaining of air supremacy should not be the hardest part.
Far harder is working out whether the longer term objective is to create an autonomous zone in the east of the country (AKA the Kurdish region of Iraq post 1991) or to keep attacking Libyan forces from the air until Qaddafi is taken out or suffers a coup of sorts (UNSCR 1973 rules our any ground/occupation presence). This is quite unpredictable and could leave us with an air overwatch for sometime (we stayed over Iraq from 1990-2003, remember)
In addition, over a longer term Gaddafi's response to limitations on its sovereignty are unpredictable. Remember Gaddafi in the past was responsible for the largest ever arms shipment that the IRA received and of course the blew up Pan Am plane over Lockerbie. Over the past week Gaddafi promised that "If the world gets crazy with us, we will get crazy too. We will respond. We will make their lives hell because they are making our lives hell. They will never have peace." In short the near term impacts of the resolution may appear clear but as long as Gaddafi remains in power the medium/long term impact of this decision could have a host of impacts that we have yet to discover or predict.