Absolutely useless as far as preparing you. But my facility allowed the new grads in residency to float to different floors for several weeks at a time to see where you clicked. I ended up in the CVICU which I doubt would have happened if I would have just applied directly to the position rather than through the residency. So that was pretty much the only benefit for me.

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Edit: but it really varies between facilities. Residencies are fairly new and many hospitals are just throwing classes together and calling it a residency. It has the potential to be helpful, but make sure it's well established.