I think testimonials don’t work and are open to abuse in many ways, as the comments on the AccountingWeb thread suggest. The fundamental problem is that the person reading a testimonial knows that they are only being presented with a partial view; they have no way to validate their opinion on it without access to the broader picture.
The next generation of clients, in particular, is savvy to this and they have grown-up in a world where Google lets them assemble something like the full picture from a jigsaw of pieces of information scattered all over the Internet. They are not, therefore, going to take any testimonial in your marketing at face value.
So, why not make it easy for your potential clients to see the full picture in the first place, from your site in a manner you have some control over?
The way forward is for professionals to open as much of their work, and their interactions with clients as possible. Show people as much as you can about what you do, how you do it and what the response from your clients is. Share the ideas you are working on, share the problems that you and your clients have faced and then, only then, share the praise they have given you. Share that praise in a way that is transparent and (prima facie) obviously unsolicited, real-time and real world.