Well I bought the system, and returned it within 48 hrs. I couldn't get it to play any xvid, mkv, mov, or wmv files either via usb or dvd. I saw tons of posts on the cnet forums where people are playing these files, but it seems to be that if you have XAC at the end of the model, it's the Canadian version and it is different than the US or UK version. Even the firmware upgrades are different.
I ordered the dongle so I could stream the files from my computer, but according to the 4 techs I spoke to at Samsung, it still wouldn't be able to play xvid files. Since we use this feature more than any other part of the system, it wasn't worth it to us to keep it.
For the time I had it, I did like the picture quality of the upscaling. I didn't have any bluray disks to play to check out the picture of BD, so I can't comment on that. I found the sound lacking a bit if you like to listen to music. I turned it all the way up and it did sound good, there was no distortion, but the highest setting wasn't very loud.