I think it's alright. It can be pretty cool. Definitely impressive. Sometimes a little intimidating, though, and I don't like them to get TOO hard, myself, mainly because I'm attracted to soft. When I think hard, I usually think "masculine". Still, that's not to say that if you're hard you absolutely look like a man. For example Piper Perabo has a great set of abs, and she's still feminine, and sexy. (Though Google image search is utterly failing to provide me with a good example, right now.) But yeah, I think muscular abs can be sexy, within reason. So long as they're not TOO hard, and the girl still has a curvy or feminine figure to balance things out.
But that's just my personal opinion. Other guys might disagree.
If she has some nicely toned abs, that's hot as long as she has toned legs too. But if she's like aiming for the strongest women, that's not good. Don't go too far because all those lines and cracks look bad on women.
It's not really that important a think for me. Just so long as it's tight and smooth it's all good. I like a girl to have a nice pair of breasts and that rarely happens on a girl with defined abs since somewhere along the way to getting cut abs, she loses her breasts. That's not a good trade.