It's the extra features that will sell the 5D Mk2, and the main talking point is of course the HD video mode. It's not the first digital SLR to offer HD video recording; the Nikon D90 had that honour, but it's the first to offer it with a full-frame sensor, and also to provide a socket for plugging in an external microphone, which as far as I know is not found on any other still camera. Movie clips are recorded in the Quicktime .MOV format, and are limited to 29 mns 59 secs, although you'll need a pretty big CompactFlash card to do this. A freshly-formatted 2GB card only provides enough storage space for 5mins 54 seconds of HD video, so you better place a bulk order now.
Video recording operates in live view mode, and it takes a bit of setting up to get it to work. First live view has to be activated in the menu, and then the Live Mode focus has to be selected. The 5D Mk2 has two autofocus systems. The main one is a fast phase detection system using nine sensors in a diamond pattern, which is used in viewfinder shooting mode, and can also be used in live view mode but requires a mirror-flip to take a focus reading. The Live Mode is a contrast detection system that uses the main imaging sensor, and is quite a lot slower, especially in lower light. It is this system that is used during video recording, but it doesn't operate continuously. The AF-ON button has to be pressed to focus the camera, and if it is used while shooting the sound of the AF mechanism is clearly audible on the soundtrack, not surprising since the internal microphone is right next to the lens mount. It's behind the three small holes just below the 5D badge.
The live view mode itself is better implemented than on some other recent cameras with this feature, with the option to turn off exposure preview simulation. This is very useful for studio work when using external flash and a tripod, since it allows the user to compose the shot using ambient lighting. The monitor is the same 920k 3-inch screen as found on the Nikon D3 and Sony A900, but Canon has added a triple-layer anti-glare coating and automatic brightness control, so it is easier to see in bright sunlight. The brightness control can also be set to manual if you need an accurate exposure preview.
Other improvements are more subtle. The viewfinder has a slightly better frame coverage than the 5D, with 98 percent view, and it now has more information displayed in the finder, including ISO setting, but although it is a nice bright finder it's still not up to the same standard as the Sony A900. The covers over the various connectors on the left side of the camera body have also been revised and are easier to use, and the connectors in question now include an HDMI socket for connection to a HDTV for still and video playback.