I often use them in compound keys (e.g., in a flat key space as might be used by a cache or similar simple key/value store). IMHO, they are superior to other common separators like colons, dashes, etc. because they are (1) semantically appropriate and (2) less likely to be present in the constituent pieces of data, especially if the data in question is already limited to a subset of characters that do not include the separators, which it often is (e.g., a URL).