Cut and pasted from the answer I gave the last time someone asked this:
You really need to see http://www.sheldonbrown.com/cantilever-geometry.html
But in summary -
- The closer the angle between the yoke wire and the horizontal is to zero, the more the force from the brake lever is multiplied in moving the pad inwards (the mechanical advantage aka MA) but the less it will move inwards for a given amount of lever movement
So a flat straddle wire brake can be easier to use if you have low hand strength (perhaps from exhaustion after lots of braking)..
..But you'll have to trade off rim clearance, because the pads will move less for a full lever squeeze
...And/or you'll lose absolute braking maximum power - because the further the cantis move the more pads are squashed and it is the degree of squash that determines absolute braking power at the rim. I.e. there is a conflict between the ease with which you can get maximum braking power, and what that power will be.
So it there isn't a single optimal angle but a continuum, reflecting demands for ease of braking, maximum force at the rim, and clearance. The retro mountain bikers who still use cantis tend to go for a low straddle low rim clearance set up with high MA and high maximum braking. My impression is that crossers tend to go for lots more rim clearance - because cross races are traditionally very muddy - and much less MA.
If you do go for a low straddle to maximize MA, then you should probably fit one of those fork widgets that catch the straddle cable if there's a break - otherwise the straddle can catch in a knobbly tyre and that can be the endo you. So to speak.
Finally, more power to stop the rim can easily be more power to skid rather than brake - narrow cross tyres have much less grip than 2" mtb tyres and grip is the real limit on braking. People confuse mechanical advantage and braking power all the time, but it is actually at least two steps removed.
Edited to add:
As Sheldon Brown says, a common mistake is to think that cantis should feel hard and definite. The opposite is true. Brake levers connected to cantis should feel soft and squashy. The squashy feeling comes from the brake pads being squeezed between the canti arms and the rim of the wheel. The more the pad is squashed - the squashier the lever will feel - and the better the braking will be. Never try to adjust your cantilevers so that they feel hard!