Are the words added to User Dictionary case-sensitive?

Yes, the words added to a personal user dictionary are case-sensitive as the system compares the exact string in the user dictionary with the word entered in the text field. However, an added lowercase word will be considered correct also while writing starting with the capital letter. For example, you added the word 'xxxxxx'. Thus both its writings will be correct: 'Xxxxxx' and 'xxxxxx'.