Feedback whether it is positive or negative will always help you improve your business. Never, Never, Never. Never make- up your own testimonials. This will only hurt you in the long run. It is unethical and dishonest.
People also can usually see through a phony testimonial.
You should also give more information about the person giving the testimonial. They will be thinking that the copywriter may have pulled those letters out of a hat. I would probably be added to 100 junk mail lists. But, if your customer has a web site, they will be more than happy to give you permission to link to their web site below their testimonial. Adding their web site link below their name also makes the testimonial more credible.