From all accounts though, I've heard that the 5D's focusing screen is better, brighter, easier to use than the 5D II's even with the Eg-S focusing screen, so I'm not really all that worried, I'm sure Canon wouldn't make a focusing screen non-user replaceable if it wasn't at least good enough to focus with their top lenses manually,
You thought the first reply was 'massive'? Read on....
'Better, brighter, easier to use' are pretty subjective, and in fact, that's probably the viewpoint of folks enamoured of their new 5DIII. The 5DIII does have a 100% VF, compared to the ~98% of the 5DII, but that's about the only difference. The 5DIII focusing screen simply is not going to show you the true DoF with a fast aperture. You don't have to take my word for it, Chuck Westfall (Canon's technical guru) addressed this specific question (link):
Q: The specifications on Canon’s website tell that 5d3 has a fixed focusing screen, like 7d, while 5d2 has an interchangeable one. How is this fixed screen compared to the standard screen and the Eg-S screen for 5d2? Does this fixed screen on 5d3 show precise focus regarding to depth of field on higher apertures than f:2.8?
CW: The EOS 5D Mark III’s focusing screen has about the same overall brightness and depth-of-field characteristics as the EOS 5D Mark II‘s Eg-A standard focusing screen. This is a brighter screen than the Eg-S with most lenses, but it does not depict depth of field accurately for apertures wider than about f/4. Manual focusing accuracy using the optical viewfinder is about the same, lens for lens, on the 5D Mark III as it is on the 5D Mark II with an Eg-A focusing screen.
So, according to Canon, the 5DIII focusing screen and brightness is equivalent to the 5DII with the stock Eg-A screen. CW is correct - the stock screen in the 5DII and 5DIII is 'a brighter screen than the Eg-S with most lenses,' because if you look at the EF lens lineup as a whole, lenses faster than f/4 are in the minority. My personal experience is that the stock screen is fine for MF with f/2.8 lenses as well, but not with f/2 and faster lenses. When you get your fast prime, try the 'trick' of stopping down from wide open with the DoF preview button held down, and see where the VF starts to get dimmer - you will not notice any change between f/1.2 and f/2.5 or so, and the DoF you'll see will match, i.e. no change.
When CW says, "Manual focusing accuracy using the optical viewfinder is about the same, lens for lens, on the 5D Mark III as it is on the 5D Mark II with an Eg-A focusing screen," that's his polite, PR-savvy way of saying that MF with a fast prime is not going to be accurate at all on the 5DIII.
Fortunately, Bosman has a great point - AF confirmation will let you use the camera's AF system to correct for the deeper DoF you see in the viewfinder. Do note that you'll probably want to do an AF microadjustment on the 50L - the AF comfirmation is only as accurate as the AF system, and an accurate AF, especially with a fast prime, almost always requires AFMA.
Please note, I'm not trying to be negative about this, but I do want you to go in with your eyes open, so to speak.
Another issue to be aware of with the 50L is that is has focus shift, as does the Zeiss 50/1.4. As they say, 'it's not a bug, it's a feature' - focus shift is an unfortunate trade-off resulting from the design of most fast primes (Canon's 85L uses a 'floating mechanism' to correct for it). If you're not familiar with it, focus shift is when the focal plane of the lens changes when you change the aperture of the lens. All lenses focus with the aperture wide open, then stop down the aperture to your chosen setting as the shot is taken. In the case of the 50mm f/1.2L (and the Zeiss 50/1.4), if you select an aperture narrower than wide open, down to about f/4, focus shift means that the lens will actually focus on a point that's behind your chosen focal plane. At f/4 or a little narrower, the DoF is deep enough to mask the effect, because by then your chosen focal plane is within the DoF. At very close subject distances, the DoF is shallower, and the effect of focus shift is exaggerated (you may need f/5.6 or even f/8 to get a deep enough DoF to mask the shift).
Note that focus shift affects both autofocus and manual focus, and also affects manual focus with AF confirmation - since focusing is done wide open, even if spot on there, the focal plane changes when the lens stops down for the shot and that's what results in the back focus.
There are several workarounds to focus shift. Shooting wide open is one of them, since there's no focus shift wide open. If you need to stop down a bit (pretty common with an f/1.2 lens), the best workaround is to manually focus with the DoF preview button pressed - and that's not going to help you with the 5DIII's focusing screen. What you can do if you're shooting stopped down a little bit is use the AF confirmation, then before you take the shot just front-focus a little bit with the MF ring.
None of these are show-stopping issues - the 50L is a great lens and will produce amazing results, albeit with perhaps a little more work required to get them than with other lenses.