I wanted to say how promptly and efficiently they respond to queries, either by telephone or email. That they are friendly, reliable, professional, straightforward and easy to understand, patient (if they are trying to explain something 'techie'!) and knowledgeable. No problem so far has seemed to big or too small.
Good level of service, sometimes outside of usual office opening hours if required, which limits 'down time' to this firm so that the staff here can get on with their jobs with minimal IT disruption.