I was really surprised/appalled to find out that changing ISO on the 5D mark 3 (and probably previous versions also) does not change the exposure meter in real time. To see the change in the exposure meter when changing ISO, you have to re-meter by half pressing the shutter..... I asked the sales guy at the camera shop who was a pro himself and he said that that's just the way it was and agreed that it was not as fast as Nikon bodies. Nikon's dual wheel system for aperture and shutter speed and the live real-time exposure meter has made it so easy and fast for me to shoot in manual. Is there something I'm missing or do you just get used to the way Canons are set up? My only previous experience is with an old Rebel T1i which I was shooting in manual almost exclusively close to the time I upgraded to D600.
On the 5D3, there are two common ways to change ISO. If press the ISO button on the top panel, you are essentially entering a different "mode". Like a separate menu system, here your only options are to change ISO or Exposure Compensation; the half-press is just to leave the present mode (and so happens to reactivate the exposure meter at the same time).
The other way to do it is to map ISO to the Set button, which I do and it is much faster and intuitive. Now you hold down the Set button and spin the top dial, and the exposure meter does change in real time.