This is a great sonnet to look at, because in it, Shakespeare deliberately pokes fun at other poets at the time who greatly exaggerated the qualities of the women they wrote about, producing idealised damsels that were in every sense of the word, angelic and too beautiful for this world. This poem, on the other hand, deliberately and with great relish demolishes such approaches, by presenting the mistress that is the subject of the poem with a grittly realism:
And in some perfumes there is more delight
Than in the breath that from my mistress reeks.
Thus Shakespeare is deliberately playing with our expectations as readers as we come to yet another love poem that challenges what we expect. However, the point of this poem is that, in spite of the "imperfections" of the mistress, or indeed because of them, the speaker of the poem can love such a woman just as much as he scorns poetic cliches of beauty:
And yet, by Heaven, I think my love as rare
As any she belied by false compare,
Thus this excellent sonnet is really poking fun at other poets and the idealised and unrealistic way in which they described their beloved. By taking a more realitic tone, the speaker thus shows his true love for the mistress.