In some cases you may be better served by a conventional stair. Spiral stairs are ideal for second stair applications. They are not conducive to the moving of furniture or bulky items from level to level. The larger diameter spiral stair does lessen the problem, but, it does not eliminate it. A spiral stair should not be used to replace an existing conventional stair unless you use a large diameter unit and provide other means of moving large items from level to level.
For a stair that is in constant use, do not be lured into using a small spiral stair to save floor space because it is a direct trade off with foot room on the stair. Small spiral stairs are tricky to walk on, especially on descent. Your heel nearest to the center column has little area on which to step.. On larger stairs your travel path is further to the outside where the tread is wider, thus making it easier to negotiate.