The more comfortable you are with a dictionary, the more likely you are to use it. Look up some words. It's important to compare definitions of the same word in different dictionaries and see which one makes the most sense to you and has the level of precision, detail and history you need. You also want ease in finding the most common definition, alternate definitions and guides to proper usage. You want to know whether a word is acceptable in formal conversation, only slang or antiquated.
Timing is everything.
Look for the most modern words and see which dictionary has them and whether the definition is current. Language is a living, breathing thing, and you want your dictionary to be as lively as possible. Dictionaries are generally updated every year and overhauled every 10. Some publishers go for quantity, others for depth of definition. Figure out how much word count matters to you and how the books stack up against one another.