I owned a 20D before this and so my view is relative to that camera. The 5D feels very much like the 20D and 30D in that it is pretty much the same size and operationally very similar except for not having any kind of built-in flash.

Weight - the first thing you notice about the 5D is that it's a bit heavier than the 20D. Not much heavier but if you walk around all day with it you'll notice. Add to that the not insubstantial weight of one of the flash units and you're looking at a camera package that weighs over 1.2kg... and that's before you add the lens! For me that means a total package weight in my hands of nearly 2kg! Another consideration for those who take photos outdoors in public places (markets or whatever) is that a user with a 20/30D and the pop-up flash looks like a pro-sumer tourist to the subject but a 5D user with a big 430EX (or worse the monster 580EX) looks almost like paparazzi and so can arouse suspicion and so ruin the chances of a friendly smile - I've even been accused of being from the DSS or the tax office by some with a guilty conscience...

One thing to note is that the new "picture styles" that replace the presets on the 20D have a marked impact on final image. If you don't read the guide and just assume that the "Neutral" or "Faithful" settings are the "best" then you will see rather soft looking photos out of the camera. The low pass optical filter on the sensor means that alias patterns are reduced but some digital sharpening is required. In the "Standard", "Portrait" and "Landscape" settings, the colour balance is shifted and sensible levels of sharpening applied for you. Choose the "Neutral" or "Faithful" settings and you will initially be disappointed and may have to adjust every image on the PC - as was intended for those modes. You can modify the presets and also save your own so things can be tuned to your liking. If shooting in RAW mode, you can choose whether to use the "picture style" setting from the camera or apply a different one in the DPP software - the "picture style" is just another recipe that is saved with the RAW data to be applied or ignored later. Obviously, if you shoot in JPEG then the recipe will have been "baked in" and can't be undone later.

The sensor performance is the same as the 20D with the exception of a Low ISO 50 setting. In practice, I found that the dynamic range was less in this "L" mode (it has to be enabled in a custom function menu - the same as the 3200 "H" mode). ISO 100-400 are so clean anyway that it seems a bit pointless and you risk camera shake so why bother... You can see the reduced maximum dynamic range in the DPP RAW processing software histograms.

The 20D was a bit poor at auto white balance in common room tungsten light and the 5D isn't much better - often going with a heavy amber colour cast. Switching to the fixed tungsten setting helps and in any event you can usually easily fix any cast in the DPP software with the "one-click" white balance eye dropper (assuming there is something grey or whiteish in the photo to use as a reference). A useful feature is the DPP recipe. This allows you to determine a white balance (and other settings) from one photo and apply them to any number of photos. So under the same lighting circumstances you can fix one photo in a set and then apply the same corrections to all the rest automatically.

Battery performance is still very good but the 5D does eat it's batteries faster than the 20D did - probably down to extra image processing and the bigger LCD screen (that it shares with the 30D). LOOK OUT! I had a 3rd party battery that was fine in the 20D but was a fraction bigger than the true Canon battery and it got stuck fast in the 5D - so much so that I had to prise it out with a screwdriver! Use only Canon batteries in the 5D!

There are a couple of "gotchas" though... Firstly there is some illumination fall-off at the corners of the image - but only sometimes. If you use an ultra wide lens on this body you will see illumination fall-off when the lens is wide open - it's not a defect, it's physics. You never see this effect on the 20D or 30D, not because they are better cameras but because their smaller sensors do not "see" the edges of the lens - they take their view from the central portion of the image only. Stop down to f5.6 or more and the problem is all but eliminated on the 5D. Good practice would lead you to stop down anyway as most lenses have a sweet spot of performance in the mid f range anyway. If you have to go wide open, consider increasing the ISO value by one stop instead. The sensor is so clean of noise that you can easily go to 400, and 800 still looks respectable. The 5D now has 1/3rd stop ISO steps for this very reason.

Storage - Bank on buying at least a 2Gb CF card - and a fast one at that (100x or 133x). The files this thing saves are MASSIVE - JPEGs weigh in at about 5-7Mb and RAW at 12-14Mb but can balloon to 18Mb depending on the scene (RAW files are losslessly compressed). No TIFF option is offered as the files would be so ginormous as to be silly. You'll need to buy lots of PC hard disk storage too... The JPEGs that the DPP software creates from the RAW files are bigger than the in-camera ones - around 12Mb each! A fast processor is going to be required too - 3.2GHz or better if you want to develop 2Gb worth of RAW images (about 119 frames) in less than lunch-time.

I upgraded the EF 28-135 IS to a EF 24-105 f4L IS and bought a EF 70-200 f4L IS - both very worthwhile investments and the 5D really delivers great results with these lenses.

I used to use a shoulder bag to carry my kit but after travelling around on holidays with the newer heavier lenses, I had to opt for a full back-pack. If you're a bit unfit you'd better get into training before buying this camera. Having said that, the 5D and a 24-105 is a fairly friendly combination and does for about 80% of my photos. That's why it's being offered as a package deal by several retailers now.

The DPP software has been upgraded over time and now includes useful noise reduction settings for RAW processing that allows reduction of chroma or luma noise separately. By selecting to reduce chroma noise only, you get rid of the colour noise that looks "digital" and leaves the less obtrusive luma noise that looks reminiscent of film grain - which can be a nice effect in itself. Leaving the luma signal alone also means that noise can be reduced without softening the image. This all means that I can comfortably use ISO 1000 now and get excellent results.

Discounting on the 5D is increasing, improving its value for money and while the 40D is a good upgrade to the 30D, it still uses an APS sized sensor so still doesn't compete with the 5D. If you want a full-frame DSLR for less than the price of a small car, the 5D really still is the daddy!