On of my students who has been very strong (his marks have been extremely high) wrote about one course I teach (E304) that it "has been a challenge to most of us".
I think that this can be interpreted as a compliment to the course as it has pushed a student who is very strong rather than letting him coast through content that is not demanding enough. Of course, if the course is chellenging to him, it is likely to be even more so for others.