Had a great opportunity to use this in anger a few months ago. Working on a mapping application, where the map could only handle a few hundred items at a time,

Had a great opportunity to use this in anger a few months ago. Working on a mapping application, where the map could only handle a few hundred items at a time, but the customer account had tens or hundreds of thousands. And the user could set a realtime filter which would select an unpredictable subset of the larger collection. QA raised a ticket complaining the subset we were using (just taking the first n items from the collection that matched the filter, collection size was enough that we didn't want to double enumerate or store the whole thing in memory) was unbalanced, eg we were giving the impression that areas of the map that had plenty of items were actually empty. Switched to reservoir sampling to make sure the subset was a fair representation of the collection, with only one pass and not blowing our memory budget. And everyone you explain it to gets to admire the algorithm.