We have been using the 5D Mark 2 for just over 12 months now.
It is used for shooting anything from 'black tie' type functions to studio sittings, landscape shots to industrial shoots.
We don't use the HD video capability as we are 'stills' photographers only.
In my opinion the key elements of this camera are;
FULL FRAME SENSOR
HIGH ISO CAPABILITY
FULL FRAME SENSOR
The full frame sensor enables a true wide angle shot to be taken with a 24mm lens. I know these days you can get 16mm lenses for 'ultra-wide' and of course fish eye lenses, but bear in mind only 10 years ago, most Pro's would be happy to have a 24mm lens on their film camera, and most amateurs could only dream of owning a 24mm lens!
We use a 24-105 Lis, which gives great results coupled with this camera. At the 24mm end there is vignetting which is a consequence of the full-frame sensor.
It is worth bearing in mind that all but the very best quality lenses have a sweet spot, this is one reason APS-C sensors were originally used on the Canon D-SLR's, to negate the vignetting and 'soft' focus issues caused by the digital sensor at wide angle, only the sweet spot of the lenses were available.
If you move to a 5D Mark 2, you will have to be prepared to invest in some quality glass. Whilst the sensor is superb in terms of image recording quality, it also highlights lens quality issues.
HIGH ISO CAPABILITY
As I mentioned we shoot 'black tie' functions, and this camera comes in to it's own for this type of work.
The light levels are invariably low, and quite often mixed light sources (especially if there is a disco set-up). We have used up to ISO 3200 for capturing images, and then printed these at A3 for clients with no noise issues on the print.
Again we use 'L' lenses, normally the Canon EF 24-105mm f/4.0 L IS USM Lens
and the Canon EF 70-200mm f/2.8 L USM Lens
, and the quality of shot in these low light conditions is more than acceptable for selling of images and prints.
Adversely, if you put the ISO down to 50, the image quality is stunning, some of the best I have seen from a digital SLR.
The magnesium body feels sturdy in your hands, the rubber socket covers are well made, and after 12 months we have had no issues with ingress in to these sockets.
The camera is comfortable to hold, and is used by myself and my wife, who also says the camera is comfortable.
One niggle I have is that the ISO button is not in the same place as on the 1d, Canon have moved the 'backlight' button for the LCD screens there. After 12 months, because I use the 1d as much as the 5D Mark 2, I am still occasionally pushing the wrong button to adjust my ISO when actually looking through the view finder. If you will only be using the 5D, this will not be a problem.
The Digic IV processor is leaps and bounds ahead of the old Digic III. Tonal range recorded is excellent. We convert a lot of our images to black and white, and without any real post processing, we are getting great neutrals in the conversions. With some minor tweaking in Adobe CS5 Photoshop (PC)
the results are very good.
Colours are well represented, although I am yet to see a digital camera that records natural greens (grasses etc.) as they were, they invariably require some tweaking, especially for printed output.
If you are Pro or Semi-pro, this is definitely the Canon camera for you. It is well built, not too heavy, and is capable of producing great quality images when coupled with quality glass.
If a very keen hobbyist, you will not regret it, but avoid pairing third party, or lower end Canon lenses with the body.
If you are in to sports / wildlife, you would be better with the 7D (or 1D mark iii or iv if you can afford it). The hobbyists would be best to go for the 50d, as it's frame rate is excellent. I still use a 40D as a back up for my 1D Mark iii.
Portraiture and wedding photographers should not need to think about which camera to buy. If you are a canon user, this should be your work horse.