Here's what I would consider the main cons of the 5DmkII, compiled from my research of the camera when it was released and experience of the mkI:

• A lot of people have complained about the colours, specifically rendering of caucasian skintones, so images require more post-processing (just search for 5DmkII skintones on google!)

• Outside AF points are simply inaccurate even in good light. AF performance is not up to 1D/7D series in general (though you wouldn't really expect it to be).

• Slow shooting speed/mirror movement. Looking through the mkI viewfinder when taking a photo, the mirror moves so slowly it looks like the world is sliding sideways for a moment!

Having said this they are great cameras. If you're not interested in the movie mode you can pick up a good condition mkI for very cheap compared to the mkII. The only thing you really miss will be the 22mp for shooting landscapes etc. but if you shoot mostly low light / portraits you wont miss it!

There aren't that many other full frame bodies from Canon, only the 1Ds(mk I, II, III). They don't really compare for an number of reasons, but to answer your question I have a 5DmkI and 1DsmkII. The 5D is usually a backup body but there are a couple of reasons I've used it instead of the 1Ds:

• It's much smaller and lighter without the battery grip attached. When I'm travelling shooting for fun this matters.

• You can stop the [external] flash from firing and still use the AF assist lamp (believe it or not you can't do this with the 1Ds as I found out when I couldn't use flash but needed AF assist)

• It has a much bigger screen so I've preferred it occasionally when doing complicated lighting setups. Even though the 5DmkII screen is the same size as the 1DsmkIII screen, the 5D one has many more pixels.

edit: it's also worth noting that the 5D is far more descrete, looking no different to any other DSLR, wheras the 1D is going to get attention wherever it goes and this can be good or bad.