Long time lurker, first time poster. I want to thank everyone who posts useful, informed commentary in this forum.
I had wanted to get a 2014 50" this year, but due to the uncertain release schedule for that size, I purchased a 2014 49" M-Series yesterday at Best Buy. Long story short, I'm very pleased. I think most buyers will be happy with this set. I'm coming from a 2013 50" M-Series, which I purchased in February of this year but returned due to horrible backlight bleed in the corners and a screen which was not, in my opinion, bright enough. The 2014 49" M-Series corrects all of the issues I had with the 2013 model. Screen uniformity, while not perfect, is very good. No bleeding in the corners, though, as others have noticed, there is a very slight reduction in brightness in the corners. Personally, it doesn't bother me - the bleeding in the corners of the 2013 series was impossible to ignore, but I don't find the slight darkness in the corners of the 2014 series to really impact viewing at all.
One thing I noticed is that, out of the box, one of the settings causes horrific ghosting. I was initially dismayed as I played around with a few sources on Netflix yesterday, and thought that either the source material was flawed or that Vizio was using terribly laggy displays, and I would have to return it in disappointment. Thankfully, neither turned out to be the case, and the issue seems to have been caused by either the "motion blur reduction", "reduce signal noise" or "reduce block noise" setting. I haven't had time to isolate which one, but turning all three off totally solved the issue, to my relief.
Black levels are very good. The "auto brightness control" on the TV (not the active LED zones, this is a separate setting) is unimpressive, and causes noticeable and distracting fading in and out of bright areas on the screen. It's a useless setting and I immediately turned it off. I haven't noticed a huge difference between having the Active LED zones on and off - either way, the black levels are very solid and the best I've ever seen on an LCD TV. Not "inky black, deep as night" black, but very, very good performance.
"Smooth motion effect" is pretty good as well, and reduces the fairly noticeable judder on big camera pans. I've never enjoyed the effect on most TV's, but on this one I actually rather like it (on low). The downside is that on certain scenes the setting introduces distracting artifacts. It's not huge, and not super often, but certainly there. Ironically, it seems to be on scenes with less motion.
Anyways, I've only had about a half hour to really mess with the TV, so those are just off-the-cuff initial impressions. Haven't done any color calibrations yet as, unlike others, I'm not hugely bothered by the colors out of the box. Skin tones are definitely too tan, but other than that it's not bad.
Bottom line, I'm extremely happy with the 49" M-Series for the price.