I compared this with a few other testimonial widgets:
-IvyCat AJAX Testimonials
Out of these, this one works the best (I had some compatibility issues with IvyCat and Paid Memberships Pro), and has the features you would expect.
Features I expect include a shortcode to display all testimonials and a widget that rotates testimonials at random (they included a nice touch that the rotation pauses when your mouse is over it).
The only downside I found is the length of the excerpt being too small in the widget. This is easily fixed by editing the plugin, and changing this line:
echo '<div class="content">'.wp_trim_words( $testimonial->post_content, $num_words = 50, $more = '...' ).'</div>';
Change from 50 to 200 or so.