I have been 5 boxing about the same length of time and had a very similar experience. For every bad comment or insult I have had 10 or 20 good comments or people asking about my setup.

I did have one guy who started off accusing me of botting, and by the end of the conversation had cancelled his GM ticket and took one of my shaman's places in AN normal so he didn't have to queue in the dungeon finder. The experience totally turned around his view of multiboxers after seeing one in action and asking me a few questions, we still do dungeons from time to time.

I also went AFK in Dalaran one time and came back to find about a dozen people gathered around my team having a heated debate about multiboxing. As to be expected the pro-boxers were winning and making the anti-boxers look foolish. I did learn my lesson not to go afk in crowded places though

The thing I have noticed, both from my experiences and from reading the forums etc. is that it seems to be 2 things which drive the negativity:

Ignorance: Can usually be countered by education. Of we course we don't have the patience or time a lot of the time to explain everything to people(and why should we have?) but answering a couple of questions or even just posting a link can go along way.

Jealousy: Tend to be the kids I think who can't afford 5 accounts with their pocket money. There seems to be a lot of this driving the hate on the forums but I have never seen it in game.