I've been lurking on this thread the past couple of days, and I feel I should add my two cents. I recently got the M551D delivered to my apartment on Friday (Thanks Amazon!). I was running through the paces of the TV being hooked up to cable, my gaming laptop, PS3 and Wii U. The picture is tremendous for my uses. (Clear and beautiful pictures for gaming and computer browsing? Yes plz). I hadn't touched the 3D since that's not what why I bought it. (I wanted the great picture and the internet connectivity) However, I decided to do my own test on how the TV does 3D.
Vizio Apps: Once you are in the apps, you can change the type of 3D. It did this on Netflix and Youtube. Although for both cases, it does automatically turn 3D on when you view a 3D compatible video.
PS3: No manual change. It does change automatically when you have something 3D related. (I tested Mortal Kombat and Netflix on PS3. Both ran fine)
Gaming Laptop: No manual change. It does change automatically when you start playing a 3D compatible game or running the NVidia 3D Vision. Tried to play a couple of 3D Youtube videos via Chrome and the TV did not go to 3D. (However, if you change the 3D options in YT to interlacing rows, it kinda works) Did not test playing a SBS mp4 because.... well I don't have one.
Cable: I tried playing a couple of 3D Videos from my On Demand Service (FYI, I have Cox Cable). I am unable to change over to 3D so all I see is top/bottom screens.
I'm thinking that it may be some type of firmware bug. I was browsing around and came across the topic for the Vizio M3D651 TV (http://www.avsforum.com/t/1439520/official-vizio-m3d651sv-owners-thread/210
). There is a mention that the latest update took away the manual 3D switching option and that (allegedly) Vizio has a fix that will roll out in the next two weeks. Unknown if it applies to this TV or not.
So yeah, that's my two cents. It's not really a deal breaker for me since I don't really watch 3D programs all that often. (In my case, it takes a lot of visual focus to get the 3D and I can only view it for 10-15 minutes before my eyes say "NO MAS") I will say that when the 3D does work on this TV, it does work pretty good. But not being able to manually switch it could be a turn off to folks who like their 3D content.