For those of you who have or have had a 5D Mark II. What do you not like about it?
If you have a 5D Mark II and another full-frame model, in what kind of situations you prefer one over the other?
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.
I've had a 5DMII for almost 2 years now, and I would be hard pressed to find something I don't like about it. Sure, after using the 7D I wish the burst rate and the focus engine were a bit better, but these (and more) will be in the 5DMIII of course. Given good lenses, I've never been disappointed by the camera. If you do intend to process videos though, it will require a really powerful computer. Of course, one thing a lot of people might not "like" about it, is that it's a lot of money :)
I fit in the category of owning a 5D2, and having used a 1Ds3 - so I have some experience with both the full frame offerings from Canon. I chose the 5D2 over the 1Ds for a number of reasons:
As it happens, the person I borrowed the 1Ds off of does a lot of safari photography, and found that the autofocus wasn't fast enough for him.
I remember the editor of one of the photography magazines once saying to me that he'd rather use his 5D (back in 2006) when out and about than use a 1D, as he also found it more comfortable on the neck.
I don't qualify to answer it really, as I can only give the perspective of 5D Mark I owner about why I haven't yet switched to Mark II. My knowledge about Mark II is based on Internet and the annoyances I mention about Mark I come from actual usage. The reasoning:
It might sound like I think both 5Ds were a bad cameras - actually it's the contrary. It's just the question was about downsides so I won't list the upsides (although there are many).
I have 5D Mk II since last February.
Overall, I'm quite happy with my 5D Mark II.