I have the 421SR as well and here are my thoughts, after a month and a week of use.
First in regards to humming, mine did it as well, Noticable when near the T.V. (w/in 3 ft). After a few weeks of watching it for a few hours ever day. Now when I turn it on it only makes the slight hum for for 15 seconds and goes away. So really not a problem, and I feel it has to do with the power supply warming up. My house is warm but the corner the TV is located can get a bit chilly.
Secondly I too noticed the LED screen bleed in lower right corner and left upper mid areas. I only see this when the screen goes all black. I also managed to reduce this by lowering the backlight.
That being said these are the only gripes I have.
The TV has a spectacular picture.
For Good Sources it knows when to switch to 3D (PS3 and XBox) But the cable companies equipment is sh-it and we all know it so they don't auto switch. But my cable guys have some quality free 3d ondemand via HBO and stars which is damn near as good as blu-ray.
It games spectacularly on the PS3. I can't reccomend Killzone 3 enough, the graphics are fuhking mind-blowingly cool.
As far as settings I am still fine tuning this myself. I will tell everyone to turn off the auto-dimming and smart dimming. Put the smoothing on low. I left film mode on.
What I am trying to make precise is the backlighting, mine is in a semi well lit upstairs livingroom. I am finding the backilighting comes way to high on all the settings. I am trying to reduce backlight for brightness and have been seeing some pretty good results. I have no real settings to give yet, but will toy with some of the suggestions and compare them to what I currently am running.
So to conclude pretty damn good tv for $600 bones. Plus I have like 20 pairs of theater glasses so everyone gets to rock the 2012 Winter Xgames in 3d with me.