Bought as a package with the EF 500mm this is currently the top Canon sports/wildlife camera. 16.1Mpixels is a little behind the 18Mpixels of many of the other current Canon cameras, but they are 16.1 very good MPixels.
Someone coming to this camera from a P&S or Rebel background would probably think it was like trying to take a photograph with a lead brick. Luckily we've been using semi-pro bodies with battery grips for a few years and there isn't much difference in size and weight compared to eg a 50D+grip. There is certainly a refined feel to the ergonomics compared to standard sized bodies+grips and I find the 1D sits in my hands very comfortably, particularly in portrait orientation.
The first things we noticed from our previous bodies were the size of the viewfinder and lack of a control dial for setting modes.
The bigger viewfinder was certainly welcome. It gives a larger, brighter image and some increased distance between the camera body and eyecup (so you don't have to squish your nose up against the back of the camera so much). There are a few minor changes to the layout of the information overlays but nothing that takes too long to learn.
The lack of a control dial isn't a bad thing per se, it mostly means just learning a new set of buttons to choose modes and I expect it makes weather sealing the body a lot easier. Losing all of the 'creative' modes isn't a big deal either, we very rarely use them these days. The major loss is in choosing registered settings. On our 50D we can set up to two complete camera settings and switch between them with a twist of the dial (the 7D has three settings). The 1Dm4 has the ability to have an infinite number of settings registered on memory card but unfortunately it requires going into the camera menus to choose them. This takes time and takes your eye away from the viewfinder. Those few seconds can be the difference between getting a shot or not and is extremely frustrating if you're used to using C1/C2 on lower models.
The theme of extreme customisation continues throughout nearly all of the camera settings. There is an almost overwhelming range of custom settings and configurations which can take a while to get right for your shooting styles. This isn't helped by some of the settings being named differently to those found in eg the 50D or 500D, unnecessarily so IMO.
Once you've got it all configured the camera is, however, very impressive and it feels like you're fully in control of it. Bought mostly for wildlife photography the requirements are for fast and accurate focusing and high frame rates, coupled with the ability to work in low light conditions. The 1Dm4 excels at all of these.
Rattling off shots at 10fps increases your chance of getting that one good shot, although it does also increase your chance of attracting attention (both from birds and birders). We tend to use the 'slow' mode at 5ps. This is fast enough to catch most action but slow enough to take a single controlled shot. The addition of a 'silent' single shot option is welcome. It isn't truly silent but certainly a lot quieter than the quite loud normal shutter sound.
The 45 high sensitivity focus points give much more fine choice in choosing the spot you want to focus on (particularly important with shallow depth of field) and more accuracy with faster lenses. Using the rear joystick makes this a lot easier to choose, as on the 50D. There are numerous autofocus modes but generally spot works best for us, for now.
The high ISO performance, compared to our previous cameras at least, is astounding. Our 50D suffered from quite heavy noise even at ISO400 and definitely above ISO800. The 1Dm4 can give very good results even up to ISO3200, with a much more appealing noise profile. Suddenly dim days aren't quite so dim any more. The camera can also go much higher although we reserve them more for where you have no choice if you want to get the shot.
Taking both compact flash and SD cards there is a little more flexibility in the use of storage. As SD cards still tend to be slower and smaller than similar prices CF cards, we use a fast CF card for primary storage and a slower SD card as emergency backup (eg to keep shooting while swapping cards or extra space when there are no other cards). We will upgrade the SD card to something faster (45MB/s) soon which will be plenty for our usual shooting rates. We mostly shoot just RAW but you can set one card for RAW and one for JPEG. I don't see that being of much use to most bird photographers, though.
The 1.3x APS-H sensor is a compromise; both a curse and a blessing that leaves it in a photographic limbo IMO. Being larger than APS-C its gathers more light and so tends to have lower noise and shaper images, but loses some of the focal length advantage with telephoto lenses (for ultimate reach a 7D may be more appropriate).
Being smaller than full frame it makes wide angle lenses longer (less wide) and so it loses some of the field of view advantage that a full frame has but without being able to use the EF-S lenses for APS-C cameras. For our use the extra effective focal length compared to full frame is welcome but it is limiting its use for wide angle work.
I could write for pages about all the features of the 1Dm4, and we're still learning how to use some of them. The bottom line is that it's a very solid, fast, accurate, dependable, configurable camera.