I have been using Canon for some time now. I also shoot Mamiya and with a Leaf back. But in regards to Canon, my choice for now has been the 5D which has been producing excellent files. I am more pleased with it for my uses than the MarkII.
Let me say that I am not much of a gear geek. My choices are based on what produces the results I need for my photography business. I had an interest in the Olympus system early on because of the new features and designs for digital. But I am not one to even try to things out without reason.
With that said, I did have the chance to use an E-3 and 12-60 for a couple of actual feature shoots. I have posted in this thread about that. And to repeat what I have said, it appears to me that the Olympus produced the sharpest files out of the camera that I have seen except maybe for the Mamiya/Leaf. They also have the best color and the best skin tones.
The issue of high iso, while of interest to me, is set aside by having f2 lenses in a very useable physical size and the IS in the body. I would rather have that and be able to shoot under 1600 then have lower noise at 3200 in a Canon. Regardless at 3200 the file is going to be noisy, so slower is better.
And the physical size of the Zuiko lenses is a big plus for me. I fly a lot, so if I can travel with two or 3 physically smaller lenses for a much broader range as compared to the big Canon lenses, that is good. When you are carrying all this gear, trying to get it on planes, then shooting while juggling two bodies and more gear, well.... less is more.
I have a 30D as backup for my 5D, and although a nice value, that little viewfinder is not my favorite. So a 40D comparison is not an issue for me. I really don't want it. The E-3 has a very nice size viewfinder, big and visible. And as far as full frame or sub-frame descriptions about the Olympus... it does not seem to me to be a consequential point. It is just names. For the Olympus system, it is full frame.
I like the 4:3 aspect ratio, gone is cropping for 8x10 and 11x14 which at times can be a problem. And I don't know if it is the aspect ratio or something else, but I see that 3 dimensional kind of look in the photos from the Olympus that I see in medium format.
The Olympus is a joy to use and even better the photos I produced from my shoots are great. I am very pleased. I am sorry to have shipped the camera and lens back. So just into January I will decide on my changes and I would say at this point for the "35mm" size camera system you will see me using Olympus unless I get to try something else out that might offer something new and better.
Also next on my list for tryout will be the Mamiya ZD. It is hard to pass up the possibility of such an inexpensive medium format back.