I totally agree with Owlie that it all depends. In general, women tend to have longer legs and shorter torsos, so most WSD bikes usually have shorter top tubes relative to the men's/unisex bikes of comparable size and the WSD bikes are available in smaller sizes. I've had six road bikes in the last 2 1/2 years, four were WSD and two weren't. The first was a WSD 45cm Kona Lisa RD which was comparable to a 48 cm Specialized Dolce and just too big for me since I'm only 5'2". I was never comfortable on that bike, probably because of the size, so I sold the Kona and bought a 44 cm Bianchi Vigorelli (not a WSD bike), a smaller bike with a steel frame compared to the Kona's aluminum frame. The Bianchi was slightly too big, but after swapping the stem and handlebar, it fit well enough for me to comfortably do 50-mile rides, and I've ridden it for the last 2 1/2 years. I just recently got a 44 cm WSD Bianchi Eros Donna with a shorter top tube that fits even better.

Once I started riding hillier, 50+-mile rides with my husband riding his carbon bike, I wanted a carbon bike to keep up more easily. I test rode a half dozen carbon bikes (Colnago, Specialized, Cannondale, Giant, Ridley, etc.), including a few WSD bikes. I really liked the Colnago Ace (43 cm non-WSD), but it sold while I was trying to decide if I wanted to pay that much for lower end components. Since I have a thing for Italian bikes and because it was the most fun to ride, I ended up getting a unisex XS (47cm) Wilier Izoard XP. Again, I swapped out the handlebar and stem, and I could easily ride the Wilier up to 50 miles with no problem. However, when I started doing more metric century (100 km or 64 mile) rides, I started getting knee pain in my right knee. My PT figured out it was an overuse injury, probably due to my carbon bike being slightly too big. She recommended I get a smaller bike, or I'd keep experiencing the inflammation in my knee.

I researched carbon bikes and found out the smallest, non-custom carbon bike was the WSD Colnago CLD size 40s. I've always drooled over the highest-end Colnago bikes hanging in my LBS, and once I got into biking, I dreamed of having a Colnago. Fortunately, I have an awesome husband who's a randonneur, so he's used to spending a lot of money on bikes. He was all for me getting the Colnago if it meant I would ride more and enjoy going on longer rides with him. In the year I've had the Colnago, I've done 5 century rides and about a dozen metric centuries, and no more knee pain. In the last month I've also noticed on the longer rides I'm finally engaging the butt (gluteus medius) muscles I'd only ever felt when my PT made me do clam exercises to address my knee pain. I never realized how much difference good bike fit makes, and now I'm totally convinced.

When I sold the Wilier, I sold it to a guy who was 5'3", so only an inch taller than me. He asked why I was selling it since I'd only had it a year. When I explained that the top tube was too long for me, he was wary of getting a bike that was too big for him since he was only an inch taller. We took the bike to our LBS, so the staff at the LBS could check him out on the bike. I actually had more clearance over the top tube than him, so his legs were shorter than mine, but his torso was several inches longer. The guys at the LBS thought the bike fit him really well and didn't think he needed to change out the stock stem or handlebar.

I think the smallest WSD bikes are critical for the most petite women, 5'2" and under, mostly because of top tube length. For riders taller than 5'2", it depends on the length of torso relative to legs. Riders with longer legs relative to their torsos will probably prefer the WSD geometry while the riders with relatively shorter legs and longer torsos will prefer the men's/unisex geometry. Then there's also the geometry differences between different brands (e.g. Specialized vs Trek vs Bianchi, etc.). That's why the best thing to do is test ride several bikes and do long enough test rides to decide which one fits the best.