It seems that people like her and dislike her for pretty much the same reason.
I think that people who dislike her, hate her for what I'll call "role-playing reasons". Her prejudice, in the real world, would be highly annoying and most good people try to disassociate from those who have bigoted attitudes. So if you just judged her as you would judge anyone else, it makes sense that you would dislike Ashley. Fair enough.
I notice that the people that like her, tend to appreciate the overall character, and care that went into making her a believable person within the universe. A lot of people wouldn't trust aliens and would be more concerned about how their own species was progressing. A lot of people with bigoted attitudes actually have a complex about their own abilities, as Ashley does, as a result of her family name being tarnished. A lot of religious people are defensive over their beliefs.
"She's kind of a bitch" doesn't mean that she wasn't a well developed character. It just means she had a difficult personality (Although I sorta feel like this has been overstated).
She's also one of the only characters that, in my eyes, has any real reason to romance (male) Shepard. Liara is more interested in the marker than Shepard himself. Tali falls in love with you for no reason at all. Miranda thinks she's better than everyone and probably wouldn't be interested in some space marine. I like Jacks character, but, she wants to be with Mr. Superman Shepard? C'mon.
This doesn't get into the fact that she simply tells you more about herself than anyone else. You find out about her family and sister, you know about at least one of her hobbies (Poetry), she tells you about her dad, religion, feelings on humanity...she simply had a lot of content.
Sorry for making this Bioware forum post here on neogaf....