Reviews add a lot of credibility to your product. Especially if they look genuine. The more detail you add to them the better. Also the more interactivity you add to the the better. Take a site like Amazon for example. Anyone that purchases a product can write a review, at any given time.
Seesmic also does a great job a putting a face on their testimonials. The add an image and the occupation and location of the real person. If there was any doubt that these people are real, Google their name and find out! Of course Seesmic has an advantage of having some potentially well kn own users but you get the point.
The thing you want to avoid is something saying "John Smith rates our product at 5 stars because it really help him be more productive." This doesn't look very credible and could have been written by anyone, such as one of the marketing copy writers (and they often are, unfortunately).
This site talks about some things to watch out for when adding this type of content, such as:
"Don't try to fake sincerity.
I'm sure you've seen websites loaded with testimonials that all sound as though they were written by the same bad advertising copywriter. Breathless, urgent, loaded with supposedly "hypnotic" sales trigger words - and completely unbelievable.
Nothing will ruin your credibility faster than the use of phony testimonials
It's perfectly okay to solicit testimonials. Ask your customers for feedback, and include the best comments on your website. However, do not under any circumstances succumb to the temptation to write your own testimonials or to hire someone to write them for you."