So I've had the 1D Mark III for a couple days now, and I've had a good chance to play with it quite a bit. I got it in an incredible deal after some wheeling and dealing on the used market, ended up trading about USD$2200 worth of gear and cash in the trade. It came mint, in box, green dot and post-fix serial number (so it was manufactured with the new sub-mirror and came with the latest firmware). Here are some thoughts after 2 days of use.
Some background: I've owned the 5D, 20D, XTi, 1D, and 1DIIN, and I've had the chance to try out the 30D, 40D and 5D Mark II for reasonable amounts of time.
To cut to the chase if you don't wanna read the details: this is the best camera I have ever used. Period.
1. Sensor size - It's not full frame
I'm coming to this as a 5D owner, so the 1D Mark III is here to form a dynamic duo with the 5D. The lack of full frame would be a disadvantage if I didn't own the 5D (since my widest lens is the 24-105), but since I do, it's a blessing for things like bird photography. IMO the 1.3x crop is a decent compromise between full frame and a 1.6x crop, since lenses like the 17-40L are still quite wide on it and you can get pretty good DOF control. Still, it's nice to have both - full frame would be hard to give up.
- The viewfinder is huge and bright. Much nicer than the MKIIN and it doesn't have the yellow colour cast that my 5D does.
A
2. The Autofocus system
This is what everyone wants to hear about eh? A few basics:
- It has 19 selectable AF points, spread all around the viewfinder, and 26 non-selectable assist points.
- ALL 19 points are cross type sensitive with f/2.8 lenses or faster, and the center point is cross-type sensitive up to f/4. The 18 non-center points are horizontal line sensitive up to f/5.6, with the center point horizontal-line sensitive up to f/8 (which means it will focus with my 400 5.6L + TC very well).
- The Mark IIN in contrast, had 7 cross type sensors clustered around the center, and the other 36 selectable points were only horizontal line sensitive. The arrangement of the selectable points on the MK3 makes a lot of sense.
So how does it perform?
- First, it's FAST. It's ludicrously fast. I was taken aback by the speed because I've never used a Canon body with an AF system like this before.
- It's ACCURATE. In servo and one shot, this body has performed admirably. In low light, performance dips a little, but it still does a good job. What's most impressive is that with fast glass in low light, it's still deadly accurate and fast in servo even with the outer points, though in these situations the center point is noticeably better. Still, miles ahead of the 5D and Mark IIN outer points.
- The performance of the outer points dips substantially in low light when you use a lens slower than f/2.8. With f/2.8 and faster glass, performance is still impeccable.
- The biggest surprise? My 85L is much, MUCH faster on this body than it is on my 5D and every other body I've used that lens on (including the MKIIN). I feel like I could use that lens in Servo now 
. Very, very impressed.
A+
Improvements from the Mark IIN
Apart from the AF, this body has changed a lot since the IIN, with some much needed upgrades. It feels like a whole new class of camera.
LCD:
- Very big. Very useful. Very bright.
- Resolution could be better. What I don't get is why zooming in with Live View gives me tack sharp focus check, but zooming in that far in the final image is close to useless for critical focus checking. The embedded JPG is just not high enough quality.
B
Live View:
- As mentioned, very cool for checking focus, serves as instant mirror-lockup.
- No AF in Live View, unlike the newer models.
B
Handling/menus:
- HUGE improvements here. It's a lot lighter (thanks to the new battery, more on this later)
- No more finicky two-handed operations.
- Menu systems make a lot more sense, and my menu is just brilliant
- The addition of the joystick and SET buttons were much needed upgrades, they've made operation much simpler and streamlined than on the MKIIN.
A+
Performance:
- Battery life is RIDICULOUS. I've shot about 500 frames over the last 2 days and it's still at 77% (the new battery check is great), and it was at 91% when I got it!
- Same zippy speed and response time as the IIN, same lack of shutter lag. Very nice. Writes very quickly, and it has a huge buffer, more than enough for any conceivable use I can think of for myself.
A+
Image quality:
I guess this is a big one for people too. I have very little to say about this except 'fantastic'. Files are detailed, with nice colours and good tones. AWB is about as good as other Canons I've used (i.e. sometimes great, sometimes crap).
High ISO performance is terrific. Perfectly usable at ISO6400 (which is CRAZY) and I'd say it outperforms my 5D at higher ISOs (1250-3200). Below that it's hard to notice because both are superb. We've come a long, long way from the 1D classic with this camera in terms of IQ.
A+
Conclusions:
It's weather sealed, has awesome AF, handles like a gem, is stupidly fast in every respect, and spits out incredibly high quality images at all ISOs. What's not to love?
Cons:
- The embedded JPEG could be MUCH higher quality; the amount of detail at 10x zoom in live view proves that the resolution on the LCD is enough (though it could be better).
- No AF in live view.
Seriously those are the only cons I can think of. This is the best camera I have ever used - the only Canon body I've liked significantly better than my 5D, and the only camera period that I've liked better than the Nikon D700. It feels like a completely different class of camera than the 1D Mark IIN was, and I mean that as nothing but glowing praise.
Highly Recommended.
Gosh I love this camera (in case you couldn't already tell) 
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). I just got my 1D a few days ago, and it's everything I lusted after when I didn't have it. Now that she's mine, I'm in LOOOOOOOVE.
