I have them on a couple old bikes, one is a 45 lb. Schwinn Varsity with chrome steel wheels. With good cables and pads and the brakes adjusted for minimal pad to rim clearance they work quite well. I ride on top of the curve of the bar most of the time. By reaching under the bar from the outside and gripping the turkey lever on the curved section I can stop just as quickly as using the primary levers. People with smaller hands probably couldn't do that.
Unless properly setup if you grip them on the flat closest to the stem most will have too much flex and hit the handlebar. I used a set on another build with aluminum rims, 700c X 42 tires, Jagwire cables and KoolStop pads for cantilever brakes on old Shimano side pulls. They were just as effective as the interupter levers on my drop bar MTB with the same cables and pads on Shimano front cantis and rear U-brake.
YMMV.
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