p.1 #2 · Anyone tried Prime 200 F2.8 vs Zoom 70-200 IS F2.8?
The simple answer : yes, the prime is much better than the 70-200 2.8 IS (:.
I faced the same problem, some time ago, tested both, also the 2.8 non IS zoom, and the prime is definitely better. A lot better compared to the 2.8 IS zoom, a little better compared to the 2.8 non IS zoom.
For a more complicated answer, I'll let the others contribute to your confusion (: .
p.1 #3 · Anyone tried Prime 200 F2.8 vs Zoom 70-200 IS F2.8?
That's interesting. I tried a 200/2.8 belonging to a friend for a hundred shots or so at a motorsport event and the results were not noticeably different from 200mm on the 70-200/2.8 IS. Saying that, I wasn't shooting wide open, more like f5.6 or so, so perhaps by this time the performance difference decreases?
p.1 #4 · Anyone tried Prime 200 F2.8 vs Zoom 70-200 IS F2.8?
The really weak point of the 2.8 is zoom is precisely 200 at 2.8 especially. It is a lot softer in comparison to the prime, the colors are more washed out, it's definitely less bright etc.. The big difference is until f4, and then a little bit remaining up to 5.6. But after that, who cares ? There are lighter, cheaper, with more effective IS, and better options of 70-200 that really shine in terms of IQ from f4 to all the way up - my 70-200 4 IS is one of them, why buy a 2.8 zoom that is not so good at 2.8 ? My choice : 70-200 4 is + a 200 2.8 prime. Same money, lighter setup, more versatile, better overall IQ. But that's just me (+ I use more the long end of that lens, it may not be the case for your shooting style) (:.
All I'm missing now is the 200 2 IS prime and I'm set (:.