I think the movie is beneficial. It helped me picture what Morrie and Mitch looked like and how they acted. Although some things were wrong, it was very similar to the book. The news interview in the movie was very similar to the actual one and Morrie and Mitch have the same personalities.
I also think that watching the movie during/before reading the book might get a little confusing. Some of the details are different. In the book Mitch is married and in the movie he is engaged. Besides the differences I think it can be helpful in understanding the book and characters better.