Do flat fenders work fine? There are some stylish flat fenders out there. I'm referring to those fenders that have no curvature across the width of the tire, though they do curve around the wheel. Some of these fenders seem to only be as wide as the tire.

This question is posed to people who have used flat fenders: 

Do you ever find that you get some splash, say on your legs, or the outer edges of your butt/back? Or are they completely fine?

When answering, please also tell me whether your flat fender was the same width as the tire, or a bit wider.
I can't speak as a user of them, but I can as one who has followed them. They seem to work reasonably well, but they aren't as good at protecting people behind you - the arc behind them is narrower than the traditional curved in guard. I would think that in this regard, wider would seem to be better

In this they are better than the horizontally flat guards you see, which do nothing to protect those following you. 

If you're only worried about your back and legs, these seem fine - but if you're concerned for your fellow travelers, I'd be less keen on them.