I would also say his argument is easy to follow, and it is written a concise and clear manner. As someone that is occasionally wary of both philosophy and classics, I was pleasantly surprised by how accessible this was.
That isn't to say this is perfect, especially from a 21st century perspective. Mill, for example, believes that most women would choose to be more involved in the home rather than seeking public office but considering that there are still people that believe that today, the rest of his beliefs are very progressive. Additionally, unlike many of his contemporaries he at least makes the argument that women should be given the opportunity to seek those offices if they want them - after all not all men are cut out for those positions but that doesn't prevent the entire gender from seeking them out.
Mill isn't opposed to marriage, simply in its present state. He believes that marital friendship is important, and that having men be the masters is bad for both men and women. Only partnership with an equal can lead to people improving themselves and bettering themselves.
Honestly, for something that was written over a hundred years ago, it is rather impressive how much Mill's argument still holds up, and honestly, there are people that could still learn from his words. He argues for the vote for women and for providing equal opportunities. He basically compares marriage in its 19th century state to slavery and speaks out against property laws and rights. He argues against the idea of nature as a determinant in relationships between men and women because there is no way to know what nature looked like and if we go by nature, that would basically give strong men the right to rule over weak men based on might alone. Basically, he argues that nature is no longer used as a justification for anything unless it involves women.