if you know "Mandarin" you might get yourself understood in many places across China (specially in Beijing and Shangai), but there´s about 52 dialects! And in many provinces people only speaks their own dialect and they don´t even know "Mandarin". Mandarin is the "official" language learned at school, but many people don´t even know about it and never use it.
People going to high school might speak the dialect at home or with their grandparents and Mandarin for "special occasions".
It´s hard to learn, to me, pronunciation is really hard, since there´s so many tones, if you just mispronounce something you end up saying a totally different thing!
I love it though, and I love Chinese people, I´d say Chinese people are one of the most friendly peoples in the world.
