I have been using this copier in my office for the last two years and up to now it hasn't give me any problems yet except for the occasional ink refill which is usually be done by the Canon technicians.
Pros: The best thing I like about this copier is the fast warming up in the morning. Imagine the frustration when we need to copy something very urgent but need to wait for a full ten minutes before the copier can be used!! This is what happened with our old copier. But after changing to Canon NP 6551, we don't have that problem anymore because it heats up real fast - in just three to four minutes only. It also copies fast too, about 50 copies per minute and the output quality is good . Paper capacity is very good, with three reams in the main paper tray and 3 more reams in the default tray.The sorting bin is very efficiently built and it can sort up to 25 copies of copied documents at a time. The touch screen LCD is very user friendly for setting up our copying options.
Cons: None that I can think of since it has been such a faithful and fast machine.