This is what people are mad about? Really? Homer is an asshole in the first half, but I don't think overall this episode is bad.
The first half of this episode Homer is a complete asshole, not letting Grandpa pee and then when he learns his kidneys have exploded he just brushes it off.
BUT when he learns his Dads about to die he immediately feels remorse and offers his own kidney. Later he tries to weasel his way out of it but not out of greed or malice, it's out of fear, the episode even has Homer sign a document saying he can't sue if the surgery goes horribly wrong, and when he runs away he feels extreme guilt and constantly talks about how he's worthless. Eventually his guilt overcomes his fear and he goes back to the hospital.
He struggles to even get through the front door but powers through and does it anyway.
If the episode ended there I'd probably give it a 7/10, because although the way the plot is kicked off is terrible and Homer is an unlikable asshole, I do think the second half that everyone hates really saves it... but then Homer runs away again, and needs to have the kidney taken by force so I lowered it to a 6/10... and then the episode kept going and Homer got mad at his dad for taking his kidney and yelled at him to give it back which lowers it to a 5. Homer also gropes Bart's kidney, the implication being that he's going to take it but that's just a dark joke to end off the episode, but even if I did take issue with it it wouldn't lower the score.
It's a shame this episode is so hated because it's really funny, and in total only 3 of its 20 minute runtime drag it down, but man those 3 minutes do so much damage to Homers character. I don't think it's EXTREMELY out of character, he's always been a jerk, but this was just a step too far. Still gets 5 stars because, again, it's really funny.