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The caterpillar in the pictures is the larva of the painted lady butterfly, Vanessa cardui. This caterpillar reaches a fully-grown length of 1 3/4 inch. It looks like your caterpillar is about there!
The painted lady caterpillar feeds on a wide variety of plants including flowers, weeds and crops. Rarely are there enough to cause serious damage, though disfigurement of a single plant is possible (as you have seen!).
More about painted lady butterfly: http://bugguide.net/node/view/2649
"Friend or Foe" is a loaded question. :) The answer depends on which you like more: Hollyhock leaves or butterflies. The hollyhock will survive and be around long after the butterfly has lived it's short life. It's up to you, but I would prefer to see the butterfly.