In Lamb to the Slaughter, does Dahl's strategy work for you? Do you think Mary will get away with the crime and should she?

On the whole, you'd have to say that Dahl's strategy is pretty effective. As the story ends, it does seem that Mary will indeed get away with murder. After all, she's destroyed the murder weapon, successfully covered her tracks, and played the role of victim's devoted wife so skillfully that the police would never suspect in a million years that she'd done anything wrong.
As to the question whether she should get away with her crime, the answer should be a resounding no. No one should get away with crime, and that includes Mary. Technically speaking, one could argue that her offense constituted manslaughter rather than murder, meaning that she lost control in the heat of the moment instead of planning her husband's killing in advance. Yet that's still an unlawful killing all the same. However, given the huge mistakes that the police have already made in their investigation, caused largely by their chauvinistic assumptions concerning gender roles, it's unlikely in the extreme that Mary will ever be brought to justice.