The big problem with a coaster brake by it's nature is that it is a rear wheel only brake. This makes it a poor stopper compared to any bike with a front brake. Even in the best conditions the stopping distance is twice what a front wheel brake or dual brakes can provide per actual tests. This is due to weight transfer unloading the rear wheel under braking whereas it increases the load on the front wheel. Under emergency maximum braking there is almost no weight on the rear wheel with dual brake setups.
Also any bike with only a coaster brake loses alll braking if it has a chain failure or the chain jumps the sprocket.
Coaster brake hub bikes are popular in Europe as city commuter/shopping bikes but they are normally ridden quite slowly by American riding standards.
__________________
Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/