To me, a good stroke drill is one that requires you to strike the cueball very accurately with varying spins and speeds. I like the old standard of shooting the long straight-in shot from the jaws (or thereabouts) of a corner pocket into the opposite diagonal corner. By varying the distance to the OB and the desired action on the cueball (stop shot, replace the OB, follow 1 diamond, draw 3 diamonds, etc.) you can make it as challenging as you want, and the bullseye is different for each variation. That, to me, is much better than shooting any single shot repeatedly, and it is a GREAT drill on a snooker table.
Aaron