There are so many problems with the IPF5000 roll units that I have added a warning to the front page of the Wiki detailing what is known about the problems:
In brief, my second roll feed unit worked for a week, now won't feed paper from the top tray when roll feed unit is installed (still feeds rolls OK, fortunately). This is the same problem the original unit had. I also have problem #3 below (discovered the exact same problem on BOTH of the defective roll feed units).
Problems documented include:
1) Gears in roll feed unit not working properly -> redesign of gears by Canon
2) Defective sensors in roll feed unit
3) Broken pivot point for paper release mechanism on left side of roll feed unit (apparently due to the fragile nature of this part and inadequate protection during shipping)
You can read all the gory details here:http://www.canonipf5000.wikispaces.com