I was reading a short story by the Japanese sci fi writer Hoshi Shinichi and came across まったく。I think it's used quite often, so I thought I would write a bit about it here, and also put up the first lines of the story 知人たち, where I read the word, for some reading practice. To contrast with mattaku is hotondo, in the next sentence.
まったく can be thought of as meaning "completely" when used with a positive, and "not in the least" or "not at all" when used with a negative. It's a very absolute word. If you have no idea whatsoever you can say まったく わからない. On the other hand, ほとんどleaves a bit of breathing room. It means rarely, or barely. I believe it's used only with negatives.
Hoshi Shinichi writes very simply. I recommend him if you want to start reading, but are unsure of where to start. He's most famous for his short stories, so you won't be discouraged by the slow reading that inevitably comes of having to look up words you don't know. To be honest, I recommend reading straight through for the first reading, without stopping to look up words or kanji. Context is very important in Japanese, so this is a good exercise for extracting that context; also, it's a confidence booster, because you understand much more than you think you do!
Here is my translation of the few lines. Try your own!
"This young man lived a quiet life in a small room on the second floor of an apartment building. He didn't socialize with the neighbors at all. He rarely even went out."