Easy interface, great customer support; fast turnaround times; easy training of new users - we are using it mostly internally but have had some success getting outside companies onboard as well! The only thing I don't like is that sometimes when you add an item into one list within TS-API (or any other platform), if there's another version available at lower price elsewhere, its not reflected until after you've finished editing all lists in question.
We're solving an issue where certain parts were either too expensive or unavailable through traditional means so this has been helpful for us.