I can't comment on the 5D3 because I passed on that one, but I have shot with a 5D2 for several years and got a D800 about a month ago.
I agree with you that the D800's live view is horribly botched. I focus manually 100 percent of the time and use magnified live view to achieve and confirm perfect focus. This works really well on the 5D2 and that is what I am used to. Being cheap, I use a Hoodman loupe to look at the LCD. My technique is to compose the shot, activate live view, press the zoom button twice to get to 100 percent, focus, then deactivate live view. That's how I've always done it on both the 5D2 and 1D4 and it works flawlessly.
So when I got my D800 I was initially very confused because Nikon does everything differently from Canon. The first issue I ran into was that the D800 stops the lens down to your set aperture during live view. Since I normally shoot at f8, this makes trying to manually focus impossible. So right away I learned I had to set the lens wide open, focus, then return the aperture to f8. This is the same technique I use on the 5D2 when using Nikon and Leica glass with an adapter, so I am familiar with it, but it is a real pain and nuisance to me.
I also noted, as you did, that having the LV zoom buttons on the left is very inconvenient if doing hand-held shots (probably not a concern for very many, but it sometimes is for me). If need be, on the 5D2, I can hold my loupe to the camera with my eye, hold the camera and work the zoom buttons with my right hand, and focus with my left hand. This is not really possible on the D800 due to the button placement.
Of course, these issues I have mentioned thus far are easy enough to work around, so they aren't really deal breakers. But the way Nikon botched live view as far as the displayed image quality is very nearly a deal breaker. When compared to the 5D2, there is a night-and-day difference in what you see on the LCD of the D800, and the 5D2 is the winner here. I find it very difficult to focus using live view on the D800 because the magnified image is super slow to update (making any camera motion a real problem) and because the image is just plain fuzzy and jagged looking. Anyone used to the 5D2 will immediately notice how poor the D800 live view image is.
For most people, this probably won't be an issue, but it is for me because I never use AF and I always use zoomed-in live view to get the focus exactly right. It can be done on the D800, but it is SO much harder to do than on the 5D2. I don't know the technical details of why this is, but I simply know that it is a big let down and something Nikon should address.
On the positive side, there is no contest between the images from the D800 and the 5D2. The DR and resolution of the D800 are very impressive, and that's coming from someone who thought the 5D2 DR and resolution were impressive (having come from a 1Ds before that).
As to the 5D3, it was of no interest to me due to the price. I love the 5D2, and at ~$2K, it is still a good bargain. Had Canon not been so greedy, and priced the 5D3 at $2K instead of $3.5K, it would have flown off the shelves, including into my hands, despite being no match for the D800. I really hope Canon wakes up and responds with a body that will compete with the D800, but I fear they have placed SO much emphasis on video (which I have absolutely no use for), that they don't see the landscape market as any big deal.