If you're still undecided, the gist of it is that she's willing to do anything he wants to pay him back for taking her in, and terrified of being sent away again. He never threatens to do so, he's manipulative instead, giving her work that's too hard for her to convince her that the best way she can thank him is sexually. In my opinion, the worst things he does are splitting her up from Anna and hiring a couple of goons to attack him to scare her (this replaces the car accident). She never realises that he's anything other than wonderful.
If you like Melinda, there's quite a bit more of her in the dark-side route, and her interactions with Claire are pretty positive. She's not giving her the best advice, but I get the feeling that she believes a lot of it herself. One thing I definitely do like about the dark-side route is that you don't have to choose between Claire, Grace, and Melinda; you can have them all. I only played the light route myself at first, but while I prefer the light-side route, I still like the dark route; I'm certainly not trying to put anyone off, just make sure you know what you're getting into as it is very different to the light route. I would certainly recommend trying it at least once if you aren't completely put off by the summary above.