I went for the f/4 IS first and then sold it in favor of the f/2.8 IS. Why?
First, I threw the non-IS versions out of the equation because I think IS is far too important in a tele-zoom to not have it.
The f/4 IS version is an absolutely FLAWLESS lens - no worries whatsoever about anything. Top build quality, sharp straight from f/4 on, quick focus, lightweight. The PERFECT lens for travelling and daylight sports.
BUT if you shoot events, weddings and portraits (which is what I do mostly) you might just not be quite happy knowing that there's a version which gathers double the amount of light and gives you a more pleasing bookeh. So for that reason, I got the 2.8 IS version.
However, this is a TOOL! It's heavy, it's rather soft at f/2.8 (especially at 200mm), it's heavy, the IS is actually a stop inferior to the one in the f/4 version and oh, did I mention that it's HEAVY? If you want something to travel, to just carry around for a walk or something similar, don't buy this lens! save yourself 700$ and get the f/4 IS, it's sharper and half the weight! If, however, you see your camera more as a working tool, don't care about the weight and need the f/2.8 aperture, get it and never look back - it's not gonna disappoint (I think thousands of pro's who make their living with this lens are enough evidence for that).
Oh and don't let the high price tag put you off. If you really want and/or need the f/2.8 aperture, you'll probably get it sooner or later anyway... don't make the same mistake like me and lose some money reselling other versions