Do I need to read Cloud Atlas before watching the movie, to avoid being confused?

No, I don't think you need to read the book. I didn't even know what the movie was about, but I got the chance to go to a sneak preview and understood it just fine. The six story lines are only loosely tied together, in that the main character in one story is somehow reading or watching the life of the main character in the story that preceded it chronologically. For example, the main character in the 1930s story is reading the published diary of the main character from the 1850s story, and then the main character in the 1970s story is reading letters written to the main character in the 1930s story. It's really watching six separate stories that just happen to be interspersed.

That being said, I would only rank the movie as OK, not fabulous. It's fairly long, too.

But I totally know what you mean about Tinker, Tailor... I saw that movie after all of the accolades, and I was not impressed at all! There were too many characters, and the plot was so convoluted that it gave me a headache. (This is saying something considering how much I like complex plots, like L.A. Confidential.) I don't know if reading the book would have helped me follow it better, but I definitely didn't like that one.