Hello everyone. I'm new here and my opinion probably does not matter, but being a 2.8 owner i wanted to help the TS out.
(1) The f2.8 USM L 70-200mm is an IS lens. For large lenses such as these, the IS is vital. It really can make the difference between getting a good,sharp image and a bad,blurry one.
I highly recommend the f/2.8 IS as the focusing is super quick and the heavy lens makes the whole system sturdier.
(2) If you are a full grown man, you should have no problem carrying this lens unless you're really weak. I walked 7 km today with the camera and lens both hanging from my neck , and i don't feel any pain whatsoever. But maybe i'm just big and strong for my age (17) , 6 foot 1 inch and better than average frame. However, this lens has great build quality and definitely worth the extra cash and load. This is the best lens i have used, and when i bought it i compared it with the f4 and the f4 did not stand a chance. I highly recommend you buy this lens. It is definitely worth the 2800 dollars or so.