Well after much consideration, my original target was a 70-200 f/2.8 IS, then after a lot of thinking and research, I reset my goal and it is now to own 70-200 f/4 IS, beside the obvious price difference, I realize that the being half the weight of a 2.8 means a lot. And the smaller size and lighter weight will make it comfortable for use over long period. Besides, it seems f/4 is SHARPER then f/2.8, last time I dont believe this but after going through, threads and threads I decided that the f/4 IS is the next lens for me. Well Im not saying the f/2.8 is bad either, but unless you are getting paid professionally for your work, den the f/4 is a worthy contender.
Some ppl owns both f/4 IS and f/2.8 IS, they use f/2.8 IS during paid low light events, other then that, f/4 all the way.