Absolutely important. I've been teaching Turkish to one of my friends in London for about 8 months. He used to make dozens of grammar mistakes and that was killin me. Then,I suggested him a grammar book and then we decreased the numbers of mistakes. It's vitally important.You cannot understand anything if you ignore the grammar when you're learnin a new language. Greets from Turkey ;)

Of course it is. If you don't understand the grammar of your own native language, it will be much harder for you to understand and learn a foreign language.

Of course it is important. One cannot say they know a language until they learn the grammar rules. However, if you only need to be able to say a few words, phrases, then I suppose you could be less rigurous about that.

Yes , it's important .

Absolutely not important. No one should start "learning" new languages by opening a grammar book. It becomes important when you start to write the language, but before that you are free from the rules.