Well, let's see if the dust buster works. Or if it just breaks down and has to be replaced after six months!
A lot of people prefer the f/4 version over the f/2.8 version because of its size and weight. Some even own both lenses and use the f/4 lens when travelling and for other occasions where f/2.8 isn't necessary.
The 70-200/4L IS weighs 760 grams, measures 172mm length, and has a filter diameter of 67mm. The 70-200/2.8L weighs 1310 grams, measures 194mm in length, and has a filter diameter of 77mm. So the 70-200/4L IS obviously has a considerable size/weight advantage, even with IS. Plus, IS is a considerable advantage for handholding at slower shutter speeds. Additionally, as someone who already owns a 70-200/4L, I think 200mm is is as short as I would go on the telephoto end and I would not want it to be cut down to 150mm. But I also own the 100-400L IS, so maybe I'm spoiled by the ability to zoom all the way out to 400mm with that lens. So a telephoto zoom lens that only takes you to 150mm isn't very attractive. Besides, a 70-200/4L IS makes a nice complement to a 24-105/4L IS or 17-85 EF-S IS.
------------ Sounded like you [a] want to fill that gap on a 1.6x body now, but want to develop a range that will work on a FF body later, too ...?
The weight and size is an issue. I would not wnat to buy a 70-200 2.8 while I would be happy to carry a f4 - only thing is that I hope street price settles at around 900-1000 which is sensible seeing what the non IS version costs.
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