I don't really think they'd say anything else, no matter what their actual experience had been.
Can you picture Jim Lewis, or any other CEO, standing up in a meeting and saying, "Man, we really messed up that decision! It's been a disaster from day one, and we have no clue how to get out of this mess!"
That said, over the years, I don't remember a whole lot of complaints here on the DIS about either system. I've never exchanged with either, but other than an occasional "no availability" or "they don't have a resort where I want to go" thread, I haven't heard many complaints.
I have heard some experienced timeshare hands say the quality of resorts was better with II, but RCI probably has more choices. For an occasional exchange, probably either one is okay.