I have a lot of camera, as a professional photographer I use 2 bodies regularly and have a few spares which are usually older bodies I never got round to selling. The mk3 was my workhorse camera but now it's no longer holding that position.

It is still a very very good camera, and you can buy them for a little over £1000 these days. For that sort of money (which is about the cost of a 5d2) you get a lot of camera for your money, but also lose out on some potentially important features.

Noticably lacking on the 1d3 is video, this is purely a stills camera (but it does that job wonderfully). Also lacking is super high ISO (it tops out at 6400), this could be a problem if you're finding your competitors are still taking shots when you've decided to pack up and go home, but up to iso 6400 it does a great job and the noise performance is only a half stop to a stop worse than newer cameras. Also lacking is the high megapixel count of the newer cameras- this is a blessing and a curse.

For shooting something like events, where you might come back with 500-1000 images the small file size of the 1d3 files is great for keeping your photo library manageable, those big 5d2 files fill up a drive pretty quickly, and are slow to FTP. Of course you can shoot in sRAW or jpeg and get smaller files, but with this you don't have to set anything- you always make small files! This can be a problem if you want to make a heavy crop, or print very large, but I have printed 40x30" images with this camera with no issues, so make sure you get the crop right in camera and there's no problem.

AF is of course completely perfect and smokes all but the 5d3 and current 1 series cameras- in low light the 5d2 just can't compare, so what's the use of all those megapixels when you're not getting focus, or you're not getting the perfect moment due to a slow FPS. If you need the full frame, or the high megapixels then the 1ds3 or 5d3 is your camera, both are considerably more expensive. For the price of this 1d3 you can get a 1ds2 which is great if you want a cheap full frame with good AF. Avoid the 5d2 for professional use, but for amateur use it's fine.

The 1d3 fills a niche, but it's a very specific niche, don't get one if you don't need the 10fps, there are better options out there.