So I've been hanging with my new best gear friend the 5D3 for a couple of days and I'm pretty darn happy with almost everything with how the new mainstream full frame camera works with the older Canon Manual focus lens systems the FD and FL.
1. The plastic around the electric contacts is now designed like that of a crop camera meaning there is no longer a problem with the long aperture lever scoring the plastic like it did with all the previous full frame cameras going back to film. This is so great, now people can use my EdMika 0.5mm FD-EOS adapter in the normal orientation and not have to bend the aperture lever a bit to clear.
2. The mirror is 0.45mm lower profile than the 5D2 and classic. This means both my FL 55 1.2 and about to be released FD 55 1.2 family of lens kit almost makes it to infinity before mirror to rear lens contact. I do wish the mirror design was like that of the 1D series because these are another 0.3mm lower profile and clear everything. At least this is a step in the right direction. Unless you are shooting the horizon you don't ever really have to worry about wether you should switch to live view to clear the mirror or not like with the 5D2 which contacts at about 25 feet focus (though really, live view is a must as far as I'm concerned if you really want to get exact focus accuracy.
The only step back from a manual lens usability point of view is moving the zoom in button from under the right thumb to a dedicated magnifying lens icon button to the left of the rear screen. This is clunky especially since your left hand is busy focusing. Also I sure would have loved Canon allowing a switchable focusing screen like in the 1D series.
All in all I'm quite happy, the 5D3 is a beautiful piece of work that is going to give my 1D4 a run for the money on what I grab when heading out the front door. I did do a quick test proving to myself that I can still get more fine detail out of a shot of my 1D4 over the 5D3 because of the extra reach of the 1.3 crop sensor. This means it stays in my kit, if this didn't happen I'd be getting myself a second 5D3. I was not a big fan of the 5D2 body apart from the hard to argue with great photographic results it generated with ease (which I know is what really counts but still). The 5D3 seems to have addressed all my dislikes, many which are hard to qualify.