I'll have to be candid (and probably sound a little stupid along the way)...
I owned the 2.8 IS before, and sold it as it was underused.
All the while, I thought the performance was OK. Basically, nothing to complain. Then i started to explore manual and AF primes, and after awhile, I started getting rid of the AF primes as well.
I tried a friend's 4 non-IS version. At the back of my mind, I was expecting to be blown away by the IQ as I heard that the f/4 versions were the sharpest, most contrasty etc etc etc. Nothing wrong with the lens. It was nice to use. The lightweight and long barrel did feel funny though, and made steady holding more difficult. The pictures were OK. But the boke wide open was just that slight bit less attractive. That's when I started to re-look my old photos taken with the 2.8 IS, and wondered what in the world was I thinking! Or perhaps i just wasn't thinking.
Fast forward, I eneded up getting the 2.8 IS version - again.
But it's a love-hate relationship. I love the performance, I love the fact that I can hoot at 2.8 and get virtually razor sharp pictures. I love the absolutely seductively creamy boke. I love the IS. I love the higher percentage of keepers with the IS. I love the fact that the heavier weight actually makes this lens steadier (I have shot razor sharp pix at 200mm 1/8 sec).
I hate carrying the extra weight around. I hate the price and I hate the fact that it's a screaming white lens.
I'm seriously thinking of selling it (again!

) and getting the f/4 IS.
Anyone wants a virtually brand new 70-200 f/2.8 IS with Canon SG warranty?

Quick question: What is the f/4 IS version really like, IQ wise? I never tried it before so I won't know.