From what I'm reading on forums, most issues with MSY surround customer service in initial sales at particular outlets, i.e. usually noobs not having a clue what they are buying and walking in looking for the Harvey Norman experience. It's just an in and out parts supplier and that's why they are cheaper than anyone else / popular with a lot of computer techs.
There have been issues with warranty dealings in the past, but the ACCC cracked down on them hard and fined them big for it a few years ago.
I can't speak from much from personal experience...it wouldn't matter anyway, cause everyone seems to have their own good or bad story and it seems to differ from store to store...but the guy I dealt with yesterday was pretty friendly, even carried some stuff out to my car for me. And that's how I like to shop...no fuss, no upselling or anything...just go in there knowing what you want and come out with it.