Where do I begin? It's darned near impossible to find parts for it if it breaks. It's heavy and a pain (literally) to try to hand hold. It doesn't have IS. It as nose heavy as they come making a pro body almost a necessity to balance it out. On a pro body you are hauling around 10 lbs of gear. Because of the size it pretty much needs its own case and doesn't fit in any kind of normal camera bag. It is definitely a specialty lens, why would you buy a monster like this and shoot it at F/2.8?
All that being said. Oh those amazing images that come out of this lens no matter what camera body you attach it to. The 200 1.8 is the Elvis of lenses. It is The King. When I absolutely, positively have to get the shot then I reach for the 200 1.8 because nothing else will do. Yes. I know the 200 F/2 IS out there. I have shot it and it is a nice lens but there is a special kind of magic that an image from the 200 1.8 produces that can't be quantified and can't be captured by anything else. Creamy bokeh that make the razor sharp focus point pop out from the background like nothing else. The special quality of the images can even be seen in a folder full of thumbnails. Open a folder with images from a dozen different lenses, go to a thumbnail view and those taken with the 200 1.8 are evident.
Autofocus on this lens is so fast that on a pro body if you blink you will miss it. AF motor is dead silent so, through the viewfinder you just notice it is in focus. Eventually you don't even think about it because you just know it's going to be there. Need more reach? Put a 1.4x or a 2x TC on it and you have 280mm F/2.5 or a 400mm F/3.6. Unlike every other lens that I have ever used the loss in AF speed and sharpness is barely noticeable if you notice it at all.
You want a lens that will hold it's value? This is it. I have owned mine for well over 10 years and could still sell it today for as much or more than I paid for it. No other lens in my bag has that distinction. I started out as a Nikon shooter but moved to Canon when my gear got stolen. Nikon has made some amazing advances in camera bodies but the 200 1.8 keeps me buying new Canon bodies.
Since I have stopped traveling and shooting sports professionally I have no real reason for keeping it other than a love affair that has existed for over a decade. I will have it until I finally wear it out and can't get it fixed. Then I will put it on display as an example of what a lens should be.
No......I would most definitely put the 2 lenses threw rigorous tests and usage for 3 months before I make a decision. There is no doubt about it they are both wonderful lenses and very versatile, both having their advantages and disadvantages. Like you I will always own a 200 prime, ....Good luck and carry on with the love affair.
