At this point, a day or two of calm is helpful and Annan is right that it might help bring in relief supplies. But long term, there is little chance of this holding or being a meaningful ceasefire. Al Jazeera:
The deadline has passed for a ceasefire in Syria to come into effect as part of a peace plan proposed by Kofi Annan, the UN-Arab League peace envoy, but Western leaders have already expressed doubts about whether the Syrian government will honour the deal.
Syria told Kofi Annan in a letter that it would halt all fighting by Thursday morning but reserved the right to respond to any attack by "armed terrorist groups".
Since rebels, loosely organised across Syria's provinces, do not obey a set chain of command, there is no guarantee they will obey the ceasefire either.