I still don't see any reviews on the net of the 70-200L f/4 IS. Pom has indicated that it's not as sharp as the non-IS version, but that the IS is well worth it despite that.
If the marketing hype is to be believed, I should be able to get a shot with the new IS version at 200mm and 1/25th sec, that could/should be as sharp as 1/400th with IS turned off, ie. a 4 stop advantage for IS.
I need to see it to believe it.
I'll jump into the fray, but won't argue about it... I bought the new lens 30 minutes after trying a friends.
I found his to be a tad SHARPER than my non-IS 70-200/4, as did my friend when comparing to his. In fact, I found this new lens as good or slightly better than my last 70-200/2.8 IS. However, I also suspect that like most lenses, there is sample variation and this could explain why Pom feels his is not as good as his non-IS version -- he had a better than average non-IS and maybe only an average IS version, while my friend and I had the reverse.
As far as how many effective stops you gain... I would say somewhere between 3 and 4 stops is a good number IF
your subject is stationary. However, I did test the lens at 200mm and got "usably sharp" images pretty consistently (maybe 70% of the time?) down at 1/8th sec, or about 4-1/2 stops below the 1/focal standard(!) But to be clear, these were not perfectly sharp, just usably sharp!
By usably sharp, I mean they were good enough for a series on street imaging where I might actually use the lens that way for example, but would not be acceptably sharp for a fine-art landscape. And I think this is the point where the "how many stops" argument will never get resolved, even if examples are posted: We all have differing standards for what acceptably sharp actually means for the type of imaging we're doing.
I will add that I personally feel IS still offers a benefit for moving subjects in that there is a distinctly different perception of subject-motion blur versus camera-motion blur in an image -- and I am of the opinion the former is more readily accepted by most viewers than is the latter.
I will conclude by saying this new zoom is a damn fine lens and IMO worth the relatively high price of admission compared to it's brothers... Its lightweight and good optical performance sees that I actually keep this one in my shoulder bag at all times. I never did that with the non-IS f4 lens because it was pretty useless handheld in low light, nor with the IS f2.8 lens because it was too heavy.