I am currently using a 1D Mk IV and a1Dx. The Mk IV is the last and best of the H sensor 1 series (1.3 factor). There will be no more.
It may have some years on it now but it is hardly 'out dated'. It is in fact a top choice for the birder crowd. It has a 16MP sensor. This may not seem like a lot except when you consider the Mark IV has the ability to shoot at 10 frames per second. It is however about the same pixel pitch as Canon's studio model 1Ds Mark II. 45 point with 39 cross point AF. ISO 100 to 102,400. It has dual card slots which is a must for the pro.
If you were to use on word to describe the Mk IV it would be fast. This camera is fast. Fast AF. Fast filing. Fast shutter.
For a good used Mk IV I would want to be around a grand. Battery and charger included. It has a 300,000 shutter but I have seen known ones go well over 1/2 a million. Perhaps $1100 for a mint copy. The birders tend to keep the Mk IV higher in used prices than other 1 series.
Adorama is a real good place to buy used. They routinely under rate their gear. Excellent will be a good excellent. They offer a return policy so all that is worth something, too. If it is what you want get it, use it and stop looking for deals.
The 1D Mark IV also has a fairly loud shutter. To give you some idea of how loud it is, just turn on any professional golf tournament, PGA tour, and listen to the camera shutters whenever a top player tess off. I have found that the shutter noise can frighten some birds.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.