Would you rather trust a company talking about how awesome it is or a colleague raving about the company's work? I'd bet a lot of money you'd prefer the latter -- that colleague is more likely to be unbiased and give you a realistic understanding of what the company is like. Including customer testimonials on your 'About Us' can give prospects and leads a more down-to-earth view of your company.
Although FortyOneTwenty's 'About Us' page starts out with a classic (but well-designed) value proposition, the key part of the page is the testimonial section below the fold. By including testimonials (with pictures so site visitors can put faces to names) and a list of companies that "trust" the company, FortyOneTwenty automatically becomes more likable and trustworthy.