Also with regard to child pornography, I would not dismiss the idea of re-victimization, the idea that by viewing material involving an abused child, you are perpetuating their humiliation. It's not something that can be broken down to an individual basis - "you are directly hurting this now-adult woman by viewing the images of her abuse!" - but can you imagine being a victim of child porn and knowing that those images are still being shared/viewed by people? It's a subtle, abstract - even ethereal - thing, but I think it's a legitimate concept that can have a psychic effect on victims of abuse. (Not psychic in the supernatural sense, to be clear).
Things like lolicon or CGI are a whole other story. As is, in most cases, viewing, say, a video of some girl on stickam showing her cooch in 2004.
As far as harm to the viewer goes, that depends too. A while back someone posted a news article here where some guy was arrested for child pornography and his house was a pigsty with garbage everywhere, CDs full of images/videos, printouts of his favorite "stars" hanging by his bed, and he would spend at least 8 hours every day viewing and downloading kiddy porn, seldom going outside, never socializing etc. He was a retired naval officer, iirc.