I've had great luck with the grooved and smooth ones, but only on low RC steels. Once you get out of the mid 50's they fall way off for the most part. Although, used following a medium grit stone they can really make a nice edge in short order even on higher RC fare, though I do not ever use one in this manner.
I don't use them anymore at all, as I get more reliable edges with stones etc and it is difficult to tell how many times one can steel an edge before it is totally fatigued. Angle control is a real challenge with repeated use as well.
It is my belief they are largely a remnant of earlier times when most cutlery was low RC carbon steel. The bit of work hardening a steel imparts is actually a benefit to edge retention in that case. The grooved steels easily sharpen those metals just like a fine file might, and the smooth ones finish them off to a very refined edge, all very quickly. I find I get more angle control using the unglazed underside of a bowl or coffee cup for a stone, and 'steeling' on the glazed rim when the knife steel gets that soft, but the principle is the same.
In use, be very careful with angle and applied force, and look out for burr or wire edges.