Great plugin! I thought of a few things, which could improve its utility:
1) Contextual Edit Links - while it is possible to locate a testimonial from the Testimonial section, having an edit link next to each testimonial link would be very helpful, especially for large sites.
2) Group Management - on the testimonials listing page, there is no way to tell (at a glance, looking at all testimonials) which ones are assigned to a group or not. Perhaps add in Group as an option for a column?
3) WYSIWYG editor for source - Though this is a small issue, it would be nice to be able to edit the source with a WYSIWYG editor. I believe it is possible to have multiple WYSIWYG editors on the same WP page (I've done this before)