Both a hard boil (caramelization due to the intense often localized heat and higher boil-off) and a soft boil (possible DMS issues due to the DMS precursors not being adequately boiled off) could produce unwanted off-flavours.
My understanding is that you want an active rolling boil, with the wort/hops/break turning over from the bottom to the top of the kettle, mixing things up, and with an adequate boil-off rate to drive off DMS precursors.
My kettle has 2 electric elements in it.
With both running the boil looks like this:
http://www.youtube.com/watch?v=vtWqpxBhRco
Which I think is a bit too vigorous, and can also get messy as per:
So after the wort reaches boiling point, I run it with one element for a 'rolling boil' like this:
http://www.youtube.com/watch?v=OGX9RPce3vU