I've had my 5D about a month, primarily to take family photos especially my 4 month old little boy.
I'd heard a bit about FF being heavy etc, but I have to say the 5D looks virtually identical to my 40d size wise, and does not feel overly heavy either. I've also read a lot about the viewfinder, but I have to say I've not immediately noticed much difference to the 40D, maybe I don't pay enough attention
I've had the 5D mainly paired up with the 85 f1.8. Now I don't know if it's the placebo affect and if the images really are that different to my 40D and 50mm f1.8, but I'm loving the combination. I don't know if it's a slightly different bokeh, DOF or what, but I'm really loving what is being produced. My little one is keeping me pretty busy at present, but I still have my 40D so will try and do some loose, completely unscientific shots for people to view in a couple of weeks.
The AF seems reasonable, it does struggle a little in low light, but then so did my 40D sometimes. In good light it's good, though I've not yet had a chance to try focusing on anything moving quickly.
In terms of the layout and functions coming from a 40D it's pretty seamless, initial it looks like to just does not have the 'landscape\portrait etc auto settings on the dial'. I'll admit that I don't tend to alter the camera settings too much, I pretty much set it to raw, stick it in AV, TV or manual and off we go.
I am going to try it with my 100-400 and see what I get, again when time allows and I'll report back. In the meantime here's a Robin taken at 400mm f5.6, slightly cropped and not too much else done to it.
Al