I have an early 1970's Schwinn Paramount that came from the factory with turkey wings and Twin-Stik shift levers mounted ahead of the steering axis to the upper headset. All works great!

I've fitted the rubber hoods to several bikes with these levers, some playing around with spacer washers is needed in some cases.
On Shimano's early Dura-Ace version, the turkey lever is the main lever(!) and the regular lever acts as auxiliary. Needless to say, these require some internal disassembly/reassembly to get the hoods installed, with holes trimmed in the right places and with a very large cutout for the big retaining screw that rotates as the brake is actuated!

Bbattle suggested that today's auxiliary (cable interrupter style) "actually work", but in fact their design is compromised by a big reduction in leverage versus the main levers. All of my "turkey wing" auxiliary levers work smoothly at about the same leverage as the main levers, and provide huge stopping power with the original calipers used on the bikes that have such levers.
I've raced cyclocross using either type, and found them all to be quite good enough, providing added safety when descending steeper sections. Brake pad selection would offer more of a difference in braking performance than lever type, but the turkey wing levers require less force to actuate, all else being equal.