Having practised turnbacks as a routine exercise as a QFI in the RAF, I have always had the thought as a possibility in later life, but practices (at a safe height) in draggy light aeroplanes convinced me that it wasn't a goer. However, for the sake of interest, I investigated the technique again as a part owner of a Europa. I found that in this slippery aeroplane, it was perfectly viable given certain criteria as mentioned by previous guys above.
You must pre brief before takeoff as to the lower and upper height limits and the direction of turn. These vary considerably depending on the wind but roughly in the region of 600 - 1200 ft for this type.
Looking out in the initial stages of the turn is not good news due to the apparent slip and acceleration over the ground. Once round the problem is nearly always a tendency to overshoot rather than drop short.