Does the protagonist have to be "likable" in order for you to enjoy a story?

I like to read some of the book reviews on Amazon and I've noticed that a large percentage of people who rate a book 1 or 2 stars generally do so because they don't like the main character. I find this odd because some of my favorite books have main character's that I genuinely don't like (Tom Ripley & Victor Frankenstein come to mind), but as long as they're interesting I keep reading. So I was just wondering what other people thought about this.