I'm currently running a mini in the living room and an ATV in the bedroom. I was watching a movie the other day in the living room (mini) 50" plasma and was really disgusted by the way a scene looked.
Just out of curiosity I ran into the bedroom and checked out the same scene off the ATV on the 42". It looked remarkably better.
So... I thought, does the 50" suck that much worse than the 42"? I grabbed the ATV, took it into the living room, hooked it up, and viola, significantly better image quality - off the SAME EXACT file. Switched back to the mini to verify, yep, no contest.
Called in the wife, showed her both. Even she could tell a huge difference and she's really non-picky about image quality.
Not sure if it's something in the decoder, or if it's just HDMI vs DVI->HDMI, but the ATV is MUCH higher visual quality.