I got my PN51D7000 setup six days ago and have put in some serious viewing time since. Maybe too much.
Overall I love this set for the PQ you get at this price point. BluRays look jaw-droppingly amazing. Crysis 2 in 3D on the Xbox 360 is kinda mind-blowing, and I am not a fan of 3D in general. 720p mkv's look great.
I do not see the issue where the set suddenly lowers or raises the contrast or brightness. Usually issues like that will drive me nuts and I pick up on em right away, but of course it's possible that it's there and I don't see it. For pic settings, I am using the default Movie mode.
I know black levels are subjective, but for me they are slightly disappointing but by no means a deal breaker. I am bit spoiled by the blacks on my Panasonic PVT25, but hey, that's not a fair comparison.
Like some others in this thread mentioned, it's really only noticeable when watching letterboxed content, and a little ambient light in the room helps. In fullscreen content I hardly ever notice. Just don't go in expecting jet black blacks.
My TV does have a pretty nasty directional buzz, which is pinpointed right at my prime seating area (directly ahead, eye level, 8' away). The intensity varies depending on what's on screen -- brighter scenes tend to cause it to spike. It's unacceptably annoying, I mean real loud, and everyone who has seen my set can hear it. I called Samsung and they're sending out a technician next Monday to have a look. They knew about the issue from last year, and said that there was a fair chance that the tech would be able to repair my TV right in my home. Does anyone happen to know if that is true of the 2010 sets with buzzing -- could Samsung fix them on site?
Besides the buzzing issue, this TV is awesome. I love it.