An archive staffed by friendly and helpful people. My goal here was to view the original of a series of letters published in the late eighteenth-century in the Philosophical Transactions, and to view the original images that had accompanied one of the letters. It took a bit of work by the picture curator, but she eventually tracked down all the images. I appreciated the opportunity to compare the original letters to the published versions, as well as to compare the original (watercolor) images with the published (engraved) images. Editorial and aesthetic changes introduced by publishing processes are fascinating.
Practicalities: You can bring your camera to save yourself all the on-site transcribing/copying.