How effective are toe stretchers with hammer toe?

Not very: They are not very effective.

It depends: They may be effective for reducible non rigid hammertoe. For the non-reducible rigid type they are not so affective.

Not very. : Splinting and stretching and padding are all aimed at relieving the pain and symptoms of hammertoes, but they do not often fix the deformity completely. Often you need a small tendon release or more involved bony procedure to straighten the toe and keep it fixed.