OneLieIn, Claire's Law is discussed in another thread. As hard as it is to say it, I suspect she's right.
Firstly, when you start dating someone, it's rare that the first thought in your head is "whoa, they might be abusive". Add to that, this means that the onus is put on the victim to find out if their partner is abusive. I'd been with my ex three years before I first sought help. Claire's Law, although the heart is in the right place, just puts the onus on the victim. It also doesn't account for relationships where the abuser's ex didn't report the abuse. On top of that, had my ex's ex told me he abused her, I'm not sure I'd have believed her. I warned the woman he got with after me. six months later, I got an email, simply telling me I was right, and she was sorry she didn't believe me. I never found out what happened.