I recently had the good fortune to have several remarkable cameras in my possession at once. Let me make clear that, with only one exception, all of these cameras were on loan. There is no way I could afford them all, and even if I could, I'd still buy only what I needed and no more. That said, it was an insightful experience. What follows is a brief summary of my likes and dislikes about each one. Keep in mind that my likes as dislikes are specific to my particular needs, tastes and predispositions. You aren't obliged to agree with me and if you don't, that doesn't necessarily make me "wrong" either.
Canon EOS-1D Mark IV
Likes: Handling this camera was like driving around town in a full-sized Mercedes-Benz sedan. It may be a lot more luxury than you need but you still can't help enjoying it. Despite its size and weight it felt comfortable and secure in my hands, especially when held vertically. I've owned and used Canon cameras (including an EOS-1n film camera) for over 20 years, so the location of the controls, the organization of the manuals, and the way the camera operates all felt like second nature.
The viewfinder was big and bright. Even though there's a 1.3X crop factor, my 35mm lenses had the practically same look as feel as they would on a full-frame body. Focusing speed and accuracy were impressive even in low light and well beyond my needs as a street, people, and event photographer.
As you might expect with a camera that has a huge battery compartment attached, battery life was excellent. Canon's specs list the battery life at 1500 exposures per charge. I'd normally be hard-pressed to shoot that many in a week.
Another side-benefit of the 1D Mk IV's size and heft is the steadying effect it has when shooting at slow shutter speeds. I felt comfortable shooting with a 50mm lens at speeds as slow as 1/30 second. I'd even venture as low as 1/15 if was either that or miss the shot entirely.
Most importantly for a camera that sells for just under $5,000, the EOS-1D Mk IV delivers on image quality, especially at high ISOs. Low light photographs often look brighter and better than they appeared with the naked eye.
Dislikes: As much as I would appreciate the size, heft, and reliability of the 1D Mk IV for paid professional use, there is no way I'd lug something like this around for street or travel photography. Not only would it attract more attention than necessary, it would weigh me down a lot more than I'd like. I'd get some great photos with it though.
More puzzling is Canon's overly warm light balance under household incandescent lights. It's easy enough to correct when I'm shooting raw files but I'd prefer not to have to, especially with a camera of this caliber.
Canon EOS 5D Mark II
Likes: The 5D Mk II features a 21MP full-frame image sensor in a body that sells for roughly $2500. (I could not care less that it's also capable of shooting HD video, but that's just me.) Said body produces awesomely good photos in good light and bad. I can understand why it's so popular with landscape, wedding photographers, and anyone else who needs to produce large photographs with plenty of high-quality detail.
As you can see from the comparison photo, the 5D Mk II is significantly smaller than its big brother, the 1D. It's also 13 ounces (368 grams) lighter. That's a plus when, like me, you like to travel light. On the other hand, the 5D Mk II is significantly larger than the Pentax K-7 and is by no means a compact camera.
Dislikes: The 5D Mk II shares the 1D's overly warm light balance under tungsten lighting--a minor complaint. Two more minor complaints are the size of its raw files (they average 25MB each) and the fact that it uses Canon's 9-point, diamond-shaped focusing array, which, although accurate and reliable, is starting to feel a bit dated.
My biggest disappointment with the EOS 5D Mk II is its relatively long shutter lag and mirror blackout times. They are not slow in the absolute sense--Canon's EOS Rebels are noticeably slower--but it just doesn't feel as responsive as I'd expect from a $2500 camera.
Likes: Just like Goldilocks, who was invariably happiest with Baby Bear's belongings, I find myself most pleased that the K-7 is not only small but rugged. Consider the fact that at 670 grams (23 oz., body only) it weighs surprisingly close to the EOS 5D Mk II's 810 grams (28.6 oz.). As with the EOS 1D Mk IV, the K-7's relative heft helps with getting sharp pictures at slow shutter speeds.
What helps even more is in-body Vibration Reduction. Although I haven't found it to be as effective with longer focal lengths as in-lens VR (which Pentax doesn't offer, by the way), I seldom use long lenses anyway and it's not as if Pentax is known for its wealth of long, fast telephoto lenses.
What Pentax is known for is its "limited" series of super-compact, reasonably fast, and impressively sharp primes. Combine that with a lens mount that's compatible with hundreds of top-quality yet affordable manual-focus lenses and you have a camera that's a street shooter's and travel photographer's dream. The K-7 was my go-to camera during my trip to Ecuador last summer and, with a few exceptions noted below, delivered top-quality results.
Another plus, at least for someone so used to Canon's layout, is that the K-7's controls are so well-sorted. I had no trouble getting used to their locations and the direction the dials turn. The top display panel, though small, has big, easy to see numbers. Using the K-7 became second nature a lot faster that some cameras I've used.
Dislikes: You might think my first complaint would be the K-7's low-light performance. It's not. True, the K-7's images are barely acceptable above ISO 800, but I rarely shoot that high anyway. What's a bigger problem for me is its tendency to produce images with a slight blur at shutter speeds between 1/60 - 1/100 second. I believe the blur to be caused by a combination of vibrations from (ironically enough) the in-body VR system and mirror shock. This phenomenon is well-documented on the Internet and is most noticeable at high magnifications.
My solution has been to make sure VR is on when I'm shooting at speeds of 1/30 second or slower and off when I shooting at 1/125 or faster. I try to avoid the "gap." What makes this even less convenient than it sounds is the fact that there's no dedicated on-off switch for VR. It takes at least three button clicks to turn it off or on and if I forget I eventually pay the price.
One last niggle is that the K-7 is a bit too small. There's not enough room on the grip for my left pinky and the controls on the back are all crowded together to the right of the display.
As good as each of these cameras is, none of them has what I would consider the perfect set of features. To get that I'd have to steal a few key features from each one and blend them together. Give me something sized in-between the 5D and K-7, with the 1D's autofocus system, the 5D's control layout and menu organization, and the K-7's range of small primes and I'd be a happy camper. For now I feel fortunate just to have had the chance to use all of these excellent cameras. I'd be happy to own any one of them and I'm sure I could make do regardless of which one was on my shoulder. Dilettantes make excuses. Photographers make photographs.