According to the following article, your testimonials should be short and sweet.

 From Gaspedal.com:

Great testimonials are priceless word of mouth assets. Here are three things the best ones have in common:

1> It's short
2> It says something specific
3> It's from someone fantastic

1> It's short

You don't need a book to show potential customers that people love you. It's especially critical on the web, where a lengthy testimonial is likely to get overlooked by a busy shopper scanning your site. Keep your blurbs short -- around 6 words -- to ensure they're being seen.

2> It says something specific

A great testimonial conveys something specific about how great you are. Numbers, results, and comments about specific features are the strongest forms of endorsements. Help your customers leave better testimonials by asking them specific questions about how your stuff has made their businesses better rather than using vague, open-ended questions.

3> It's from someone fantastic

The more recognizable and respected the person saying something nice about you is, the more influential that testimonial will be. Take the extra time to actively pursue your big-name clients, event attendees, and high-profile industry analysts for their opinions on your work. While this isn't to say a fantastic review from a "regular" customer is worthless, mixing in a few reviews from well-known individuals and brands can mean a lot to a new customer.