I've had the 5D for a month or so and I'm really happy with it.
The IS buys you about 2 more stops in low light or when using a telephoto lens. If low light is a priority you need to remember that the KM series has an ISO 3200 setting (admittedly grainy, but for that shot that you "absolutly must have" it's an option). The ISO's up to about 800 are pretty clean, and 1600 is useable.
The KM's have a larger screen than the competition with the 7D being much higher resolution than the 5D.
The 6MP sensor is not too much of an issue for prints up to 20x30 or so using some of the new interpolation settings built into Photoshop CS2. They actually look pretty good.
The Down Side;
The pre-flash on the KM's flashes often causes a "lazy eye" or blinking in photos of people. It has to do with the duration of time between the pre-flash the camera uses to set it's metering and the acutal-flash used to expose the photo.
Other brands, Cannon included have a much shorter delay, and take the picture before people have a chance to blink.
There are work arounds, but the problem does exist "out of the box"
Lots more info on the KM SLR's available here:
http://forums.dpreview.com/forums/forum.asp?forum=1035