I had to sell my MK II n for monetary reasons...took a gift certificate and a little money and bought a 40D.
Here are my impressions
View Finder:
40D has a bigger, brighter biew finder than the Mk IIN
LCD Screen:
40D's bigger LCD screen is nice, but in this case size really doesn't matter....if a shot is out of focus you won't be able to tell without zooming...its a cosmetic upgrade that is nice but doesn't really matter that much.
AF:
MK IIn crushed the 40D in this area - not much of a comparison. But, the 40D still has a great AF. If you've never used a MK IIn you would think the AF on the 40D was the greatest thing ever.
Body:
Nothing feels like a 1D body....the 1D body trumps the 40D body in all aspects. But, the 40D body is very solid, big and easy to hold.
Burst Rate:
1D MK IIN - 8.5 frames per secod
40D - 6.5 frames per second.
I thought this would make a bigger difference, but it really doesn't Just time your shots better and you won't have a problem. I don't miss those extra fps at all.
Shot buffer:
the 40D crushes here - the both can take about 17 raw shots before have to download into the buffer. But the 40D can take up to 75 JPEGS vs the 1D MK IIn's 28. This is huge...I was playing around (testing) the 40D and took up to 130 shots (with a decent speed card) without worrying about it filling up. That never happened with the MK IIn.
IQ:
I know this might cause a little flaming, but to me, the 1D seems to have a little better shot right out of the camera. the 40D shots are a little soft to me, but very workable. They need a little more sharpening than the 1Ds do.
In all, I'm not sure I'll miss my 1D that much...but I will miss the AF on it when shooting my dogs (one of my favorite things to do) as they run toward me. The 40D doesn't handle this half as well.
If you can afford the upgrade to the 1D do it...the body is fantastic. But if you want a bigger view finder, LCD screen and shot buffer..go 40D
Hope this helps.