I don't find them user friendly, but I don't drop a performing sponsor/site because I have to take a few minutes to get my marketing materials. Also, write them and tell them about the color combination, font size, etc being difficult to read. I do it all the time, and many just didn't realize it was an issue and have fixed the problems.

You can also request they setup a zip file link for all of the banners and fetch them that way.