I've been playing with this set for the last few days and I've come to the following conclusion.
Native 3D content works very well. The Deep Sea IMAX blu-ray was the first thing I tried and it blew me away. Works well even at off angle seating locations.
2D < 3D is ok at best, but nothing special. I prefer just to watch the 2D content in its native 2D mode.
Gaming in 3D seems to be hit and miss depending on the source. Super Stardust HD in the PS3 is amazing and the single best demonstration I've seen yet of how 3D gaming can be immersive and mindblowing! When your ship explosed it really looks like the particles are hitting you in the face. The depth of the planet is great too and the quality is phenominal all around.
I also tried GT5, Mortal Kombat, and Killzone 3. Those were a mixed bag. Although GT5 added depth, there were common instances of seeing double images of certain areas on the tracks that ruined the experience. Killzone 3's visual quality is degraded quite a bit in 3D. Looking down the iron sites is sweet, and the depth actually helps in that regard, but I'd rather play that game in 2D. Mortal Kombat barely does anything to add to the 3D experience. Nothing really jumps out at you. Kung Lao's flying hat when blocked is the best example I could find. Gaming in 3D, with the exception of Super Stardust, has been a giant pile of fail.
I will say there is a noticeable difference in how smoothly panning the camera around in games works on the 5600 when compared to the LG PK550. Playing Uncharted 2 I immediately noticed the difference. When panning it was quite a bit more blurry, whereas the PK550 was smooth. I notice things like this though because my eyes are very experienced from over a decade of PC gaming. I can tell the difference between 50fps and 60fps easily.
As for the set itself. Viewing in a brightly lit room is amazing using the Standard mode tweaked a bit for tv shows, or the gaming mode for playing videogames. ISF's default seems pretty good for movie viewing in the dark.
Comparing the LG PK550 to the LG 5600 one after the other I immediately noticed the 5600's shortcomings. The backlight is not even at all, and that's expected from edge-lit LED displays. Even with local dimming on high, I still noticed some light-bloom around logo's when loading games, which distracted me.
The viewing angles on the 5600 also suffer. Colors really get washed out from the sides and viewing from angles of more than 40 degrees are almost impossible because of it. With the plasma, I can sit by my computer and view from about a 145 degree angle and see what's going on just fine. Not with this 5600 though.
I like the TV for the native 3D and the games that work well with it are a spectacular experience!! I now have a very difficult decision to make, and that is whether or not to keep this tv. Overall 2D viewing quality is not even close to as good as on the PK550. If I keep it I will miss the deep, dark, even blacks from the plasma and the great viewing angles. If I return it I lose out on 3D and the amazing daytime viewing. Hmmmmmm... I hate this decision!!!!