I don't know if I would call it difficult either as much as maybe a bit time consuming but then anything that deals with the level of detail OsCommerce does would be as well. I'm not one to learn very good out of manuals as I tend to remember things better by doing them (and usually messing them up a few times in the process).

Take a bit of time and play with it if you can and see if it is something that will work for you. Make sure the time it will take you to get it to do what you need will translate to time saved and ease of use to your customers.

Chris