I just got back from looking at a 64" F8500 at Magnolia in Best Buy. I too am concerned about the brightness pops since I returned a 51" E7000 last year due to it having a lot of brightness pops and would turn the panel off when a full black screen was present. I have one clip I brought on a USB flash drive, along with other clips and test patterns to test the brightness pops and various image quality aspects. The particular clip is from the Planet Earth Blu-ray, first episode, beginning of the second chapter, and exact time of 9 minutes 6 seconds. The scene is an aerial shot that passes through some clouds to reveal the Canadian tundra which is medium to dark brown and gray. Ideally the transition through the clouds should be smooth with no noticeable fluctuations. However, last year's E7000 and E8000 had very noticeable brightness pops that I couldn't eliminate within user menu settings. I also tried GT50 last year and looked at a VT50, and they did not exhibit any noticeable fluctuations.
Now the PN64F8500 I watched it on still did have fluctuations, but it wasn't as bad. This was done in Movie mode with default settings (as far as I know). I tried several different Cell brightness settings, 20, 18, 16, 10 , and 0. Settings of 20, 18, 16 looked about the same, but 10 was better and 0 was perfect I think; but who the heck is going to use 0. I wanted to test out more setting variations (cell, brightness, contrast, etc), but that was going to take a long time and I can't access every picture settings when playing media from the USB ports. Eco mode was off, any dynamic adjustments were off, but the black optimizer was grayed out as "Auto". I don't want to make judgment calls when I can't fully control the set. Since I noticed the fluctuations to be not as bad and others saying they have gotten it to be a non-issue, I pre-ordered a 51" to try out myself. Hopefully I'll have it by Tuesday or Friday next week at the latest.
As far as the panel turning off when the scene was full black I don't think I noticed it happening when I tried the Dynamic Range Low clip from the Spears and Munsil Blu-ray. I only checked it really quickly and it was hard to tell if the panel was on or off with the bright lights that this Magnolia had on. The panel turning off during a full black scene only happened on the E7000 when the brightness was below 50 or 48, I forget what the threshold was. I tried the F8500 with brightness at 45 and like I said I don't think I noticed it on the blinking test pattern. I've heard of other Samsung TVs doing this, Plasma and LED. I have a friend that has a C8000 plasma and it exhibits the panel turning off during a full black scene. It seems that Samsung thought that if the scene was black they might as well just turn the panel/backlight off to achieve even more black. The downsides to this are the very noticeable fact the display turned off and if the black scene was a quick transition, you lose a half to a full second of video due the display having to turn back on. I"ll be sure to check this out when I get my set, since I need a proper lighting environment to confirm this.
Lastly from my quick look at Magnolia, I noticed the F8500 seemed to handle plasma "line bleed" quite well. I had a couple test clips that looked very bad on the GT50/VT50, but it was near negligible on the F8500. I also had a couple stills of the dreaded green movie rating screen and I was hard pressed to notice anything. Now this was with only a hand full of clips and images and in a semi-bright room, but I was impressed. I can't wait to evaluate the set myself. I just hope I don't get a buzzer. The GT50 I tried last year buzzed severely, my friends C8000 buzzes very loudly, however the E7000 I tried the buzzing was a non-issue, so maybe I'll get lucky.