Humor is one of tools available to the essayist/rhetorician and as long as the effect of humor (light-hearted emotive response, undercutting a pseudo-serious subject, exposing false logic, etc.) fits the topic, humor is appropriate. When your essay is seeking to convince the reader to buy something (and in a sense all essays are sales essays), you naturally express its qualities; the question is really how fruitful (or damaging) is the act of denigrating competitors with humorous comparisons or criticisms. On the other hand, you want to put the reader in a good sympathetic mood toward your product. Ask yourself what function you want your humorous speech-act to perform. Also, be sure you are still introducing or summing up your "pitch". One final note: judge your audience, and judge the seriousness of the product.