I've also got the M3D420SR (purchased on Black Friday at Walmart).
I like the picture and I am satisfied with the TV on the whole, but if I'm allowed to be picky, I've got a few issues with it (listed in order of highest level of annoyance to me).
1) Glare is a major issue with this TV. I have a large window to the side of the TV and the glare makes it almost unviewable (from certain seating positions) as indirect sunlight comes through the window. Lights in adjacent rooms are also quite noticeable/annoying, especially when the picture shows a black/dark background. The TV is in the same place as my previous LCD flat panel TV, and it was never an issue with the previous TV. This TV is almost like a mirror.
2) Light leakage at the right/left edge of the TV. Especially noticeable during credits of movies when the background is black and the text is bright white.
3) Remote control is not ergonomically correct. Awkward feel in the hand and button placement could be better. This is a minor annoyance since I've already bought a universal remote for this TV and packed the original remote away. Go to Amazon and read some of the reviews on this remote (model number VUR10). Many people are complaining that the remote dies after only a few months of use. I like to keep my original remotes in tact for universal remote learning in future.
4) This may be just paranoia, but after buying the TV, I've been reading lots of complaints about Vizio's and the "pop of death." One reviewer on Amazon states that his TV was DOA and he's stuck because he bought the extended warranty, and now it is no longer valid because he has already returned it because of a dead TV. I did not purchase an extended warranty and I'm not sure how I feel about it. I typically think extended warranties are a waste of money, but I am also concerned about the lifespan of this TV, even though I know that regardless of cost, all brands have their issues, not just Vizio.
Based on my feedback, I realize it sounds like I really don't like this TV, but I actually do. I think the positives outweigh the negatives. I love the fact that the speakers are in the front and facing the viewer (duh, why are so many manufacturers putting speakers at the back of the TV?). I've got my TV in a cabinet, so rearward facing speakers just won't work for me, and I'm not interested in adding a soundbar.
Getting back on topic...I've also been looking for good calibration settings. The default settings make the faces of newscasters, etc. seem a little washed out to me. I've played with the custom settings briefly and I'll post my changes. These values are by no means scientific, as I didn't use a calibration disc. I'm hoping that someone will calibrate with a disc and post their settings here. I'm still not totally satisfied with the video settings changes I've made so far, but here goes...
My "Custom" settings are:
Not sure if it matters, but...
Color temperature: Cool
Most other advanced settings are default, but I turned off the Smooth Motion Effect, Smart Dimming, and Ambient Light Sensor.