What's your purpose for learning Japanese? How you learn will depend on your function. In terms of grammar and general syntax, it's not an overly hard language compared to other languages. The things that make Japanese complicated are the different types of honorifics, as well as the emphasis on context.
Most courses and books teach you the polite Japanese, which is good for conversation. So if you're planning to travel to Japan and want to have a few sentences under your belt, that is the way to go.
If you watch a lot of anime, they speak very informally and impolitely. It's more of an everyday speak, and is very slangy. In you want to understand that, learning polite Japanese doesn't help you all that much.
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