Good testimonials ...

When you are asked to write a testimonial, knowing what testimonials do (and don't) work, can help:
Why Testimonials Do (and Don't) Work
There are times when testimonials are incredibly powerful, and times when they might actually hurt the company's reputation. Testimonials hurt when people don't think they're real. Anything that sounds vague or cliché can smack of insincerity. Strong testimonials:

1 They are specific.

Vague testimonials don't really tell the customer anything.
A vague testimonial reads like: “I was very satisfied with the service." “I got great results." “Quality tech support." “I really enjoy your product."
Specific testimonials say things like: “I was impressed by the 24-hour concierge service, especially when they found an all-night printer at 3 am. " “I increased the number of sales leads from 2 per month to 10 per week." “The section in your course about negotiating with stubborn sellers was invaluable." “Tech support spoke to me in plain English and isolated the problem in 90 seconds." Specific testimonials work for two reasons: First, they sound more credible. Second, they promote a specific benefit or address a question that may help persuade potential buyers.

2 They overcome objections.

That's right. Testimonials that talk about objections can actually help the company. Testimonials from skeptics stand out because they sound credible. And by addressing and voicing what many perspective customers may be feeling, these testimonials are powerful persuaders.
Testimonial that address objections sound like: “I've been in property investing for 7 years. I didn't think there was anything in this course I didn't already know. But, ultimately, I was surprised to learn valuable techniques I'd never considered before." “Taking an online course felt really impersonal to me. I was afraid I'd miss the in-person interaction. What I found, though, was that the format was incredibly interactive. I've even made some friends in my study group. We phone and IM, which helps since we're facing similar challenges. It's great!"