Here's my understanding of womens' skis, as far as forward mounting and flex is concerned.
Compared to a man, a woman's hips are probably wider, her height and her legs are shorter, and she carries more of her weight lower on her body. As a result, when she stands on her skis with her knees bent, her center of gravity is lower and farther back, making it more difficult to apply the tip pressure necessary to start a turn and to stay balanced over the center of her skis.
From another perspective (Jeannie Thoren's take on her web site): "A woman’s center of gravity is on average one inch lower and further back than a man’s. She also carries more weight below the waist; around her hips, buttocks, and thighs. Men, broader in the chest and shoulders, carry their weight above the waist. This difference in anatomy accounts for a woman carrying her weight further back on her skis. . . .You can think of a man as a pear standing on its head, and a woman as a pear sitting on its bottom. When a man flexes forward on his skis, his center of gravity is over his toes. When a woman flexes forward, her center of gravity is over her heels. Skis initiate turns from the front end of the skis, so when a skier’s weight is concentrated too far back on the ski, the unweighted tip will shoot forward, wander, and possibly cross. Many women have difficulty getting forward on their skis and staying there."
Forward mounting (or a forward waist placement on the actual ski) can make it much easier to balance in the middle and pressure the front of the ski. Women's skis usually have a softer tip, too, which draws the ski into the turn with less effort. Also, women's skis are frequently made of more flexible materials than men's skis, so it takes less energy to bend them into an arc without giving up edge grip or carving performance. They're usually light weight, too, making them easier to maneuver. (This is a little different with some of the higher-performance women's skis, which do have metal in them, but still aren't quite as heavy and stiff as their male counterparts.)
In order to turn a ski, you must be able to put the ski on edge and put pressure on it. The pressure, which is aided greatly by your body weight, is what bends the ski into an arc. Unisex skis are designed and tested by men, who usually weigh more than women of equal height. In other words, if you weigh 140 pounds, you have to work a lot harder to pressure a ski than a man of the same height who weighs 160. If you're only 115 pounds, you have to work even harder.
So all of this is to say that women's skis work well for a lot of women. If you are very small (like me!), tend to have problems getting in the back seat, or even problems like a knock-kneed stance (very common in some women due to hip width issues), then womens-specific skis can be helpful. They vary in terms of how much they cater to women--some are set up for pretty far-forward mounting, other are not; some are very flex-y, others are not. Largely depends on performance level and maufacturer. That said, women who were strong skiers before on traditional skis may have no interest in some of these features. My friend who was on skis practically before she could walk and has always skied with very aggressive men HATES the feeling of a forward mounting point, even though she is tiny and tends toward a knock-kneed stance outside of ski boots. She learned to ski incredibly well on traditional, very stiff race skis, and in race boots, and has no interest in the soft-feeling, forward-mounted chick skis she's tried.
Also, as the pp pointed out, some of these problems can also be corrected with women's boots, which tend to have a softer flex and address some stance problems. Probably a lot of the skis' popularity is marketing hype, which is part of why I stayed away from the girlie skis for so long. Didn't think they were really worth it!
Just like anything else, I think it really depends on your own challenges on the snow. For itty, bitty me, who's a strong skier but likes to ski with people much stronger than I am, I have found that a women's ski is a great tool. I have a lot more control and comfort in any situation, except on the lift, when I have to look at the stupid, damn flowers!