Think of Raistlin Majere. He's definately not very likable.
He's manipulative, he's sarcastic, he's bitter, he's a survivor, he's cynical, all in all he's our darker selves.
He is however, easy to empathize with.
He has an older brother who always casts a shadow over him. Everyone loves his brother, and he needs his brother to survive. His brother is stronger than him, better looking than him, and gets all the girls. All that Raistlin has over Caramon is his brains.
We've all had someone like that in our lives (whether or not that person is male or female, or even related to us). From the viewpoint Raistlin's choices can be dissapointing, but we can never damn him for them. Although there are much nicer and more traditional heroes in the Dragonlance cycle, Raistlin stands out above them all. Even though he's not very likable.
So I would say that the reader does not have to like the character very much, but he must empathize with him.
Hell as Dalamar says about Raistlin, "'We've all been laughed at one time in our lives. We've all been jealous of a sibling. We have felt pain and suffered, just as he has suffered. And we've all longed- just once- fore the power to crush our enemies! We pity him. We hate him. We fear him- all because there is a little of him in each of us, though we admit it to ourselves only in the darkest part of the night.'"