The text plays on the expected complimentary conclusion to the phrase 'Of all the people I've met...' but subverts it with a neutral and humorously underwhelming ending, 'you're certainly one of 'em.' While it may be seen as a form of wordplay, it does not exploit different meanings of a word or similar-sounding words, which is the essence of a pun.
Of all the people I ' ve met , you ' re certainly one of ' em .