So, I've recently finished* Les Miserables, and although this character has been defended here before, I'd like to do one myself, if y'all don't mind.
Marius hate is a bit odd among bashing in that I can't really tell how much of it is genuine hatred of the character and how much of it is a big inside joke among the fandom. Regardless, Marius may just be my favorite character, and it sucks to see him bashed. Now, there are two main flavors of Marius-bashing, which I will call, respectively, 'He's a Marty Stu!' and 'He's a jerk!'
He's a Marty Stu!
It's a pretty well-known fact among the fandom that the basis for the Marius character was a young Victor Hugo. Naturally, this has led to numerous accusations of Stu-dom. The people calling 'Stu' have even gone out and gotten evidence to back them up; apparently, if Marius is put through the Mary Sue Litmus Test, he comes out irredeemable! Pretty damning evidence, no? Well, I say definitely not! Marius is a believable and human character. He has human reactions to things that go on around him; I can't really think of anything Marius does that makes him anything other than 'just a guy.' He does nothing unbelievable in the situations he finds himself in; certainly nothing over-idealized. If anything, he's overwhelmed by what goes on around him; he's hardly the biggest and loudest character in the story, whereas, if he were a Marty Stu, he would be. If Marius were a Marty Stu, he'd be the one leading the revolution, and he'd be debonair and positively sweep Cosette off her feet. He'd be kind to everyone he meets, whereas in the book he can be, while never cruel, empty-headed toward others. See all these things Marius could easily be, and yet he isn't? I wouldn't even call him a hero, really. He had his little moment when he threatened to blow up the barricade, but I don't think any of his actions are overtly heroic, or even role model-like. Does this make him a bad character, though? Hardly! He's just not a hero, though he is a romantic lead. And there's nothing wrong with that.
He's a jerk!
This one is probably the louder of the two camps. In fact, Marius NOT being a jerk in the musical was once cited as an example of Adaptation Decay over at TVTropes! The main reason behind this accusation is Marius's alleged treatment of Valjean. I, having recently read the passages in question, can say that this is all a load of hoo-ha. The passages in question refer to Marius's reaction to Valjean being an ex-convict. Having heard about Marius's apparent cruelty to Valjean before I read the passages, I was all prepared to hate him, too, so imagine my surprise when one of the first things Marius says regarding the situation is an offer to use his grandfather's connections to get Valjean pardoned (Rose translation, p. 1144)! It is only later, after sitting with the issue for a while, that the situation begins to revolt him. And why shouldn't it? To call Marius a jerk at this point is to assume that he has an omniscient perspective like we do, a kind of unreasonable thing to expect of a fictional character, no? I mean, he thinks Valjean killed a guy in cold blood, and has no reason to suspect otherwise because he doesn't and can't know otherwise, and when he does, he immediately stops thinking that way.
Perhaps related to this accusation is the oft-said "Les Amis and Eponine weren't really his friends." This isn't exactly Marius-bashing, but it's Marius-related and commonly used as a bashing tactic, to bash the musical. To this, I direct you to the chapters 'Marius, Emerging from Civil War, Gears Up for Domestic War' and 'Dream Effects Fusing Into Happiness,' pages 1094 and 1110 in the Rose translation. The narratorial freaking voice calls them his friends (though admittedly it's technically possible to interpret the first one in different ways), I don't see how it's possible to argue with that.
*by which I mean I read almost all of an abridged version, lost it, and rented an unabridged version to finish it off, if that somehow makes me a lesser being.