If you're advertising that you're the best hot dog stand in town, people will probably think you're just bragging. There are some things that simply backfire if you say them directly. You can solve this problem by having someone else say it for you, making it so much more believable, and it doesn't even matter that much who says it.
Granted, a testimonial from your local "eating out" magazine or from a renounced hot dog connoisseur may sound swanky, and everybody loves a great name (or "brand" for us marketing folks), but are these sources really relevant to the average customer? Your average customer, say a construction worker, may not really be able to relate to the testimonial by a TV chef. Think about whom you really need before asking for testimonials. Likely it will be a mixture of different customers - think of your market segments and find a testimonial for each of them (prioritize in order of market segment size).
Typically, a company's testimonials will be written by the same marketing person, based on a quick conversation with the customer. As a result, all testimonials will sound very similar in style and tone.
I've read the recommendation to keep typos and grammar mistakes in the testimonials so they don't lose their authenticity, but I would not recommend this approach for most companies because it both detriments the company's and the customer's professionalism. However, I agree that you should not over-polish your testimonials because they will otherwise lose all of their authenticity. Testimonials that are not perceived to be authentic lose all value, even worse, they can even backfire when readers feel manipulated.
If you want to read about how to craft a testimonial, I recommend the posting 5 Tips for Knockout Testimonials on Copyblogger for some additional tips on what to do and what best to avoid.