Like when buying any high end piece of equipment I debated for a long time over which way to go and looked at options from all the manufacturers when I was looking to buy my 70-200.
I discounted the Tokina straight away as while I love the build quality of there glass I couldn't find enough info on comparisons between it and the rest and didn't know anyone else who had one. In my opinion everyone gets caught up too much in test charts etc. anyway and real world circumstances are much more important. While reviews etc. are useful I always prefer to hear first hand from end users or try something myself before making a decision on what for me is a fairly expensive item.
I read many reviews on the Tamron Vs the Sigma O.S and the Tamron pretty much always came in a poor second. The Tamron seemed to be only every recommended on price and that was mainly due to how over priced the Sigma was on launch. For a couple of hundred pound more the Sigma is better built, has better A.F, better optics and has the O.S. Having had Tarmon and Sigma gear before I also know that while both can have issues with lenses, Tamron certainly appear to have more issues with back focusing etc.
Deciding between the Nikon version and the Sigma was more difficult. I borrowed a friends AF-S 80-200 and had a play with his 70-200 vr2 and was impressed with the quality of the build but hated the weight and bulk of the 80-200 but was blown away by the optics and the V.R on the 70-200.
Then the decision came down to cost. The Sigma is well worth the extra couple of hundred pounds more than the Tamron, so that was a no brainer. I decided that the Nikon was not worth more than twice the cost of the Sigma for me and eventually decided to run with it after much deliberation. For the same price as it would of cost me to buy the Nikon even
second hand, I was able to buy the Sigma, a D300 as a spare body and a 2x teleconverter and still had some change left over.
Now that I have had the Sigma for a little while I can honestly say I am delighted with it. I have no issues with it all, the A.F in real world conditions is super fast and I have not noticed any difference between it and the Nikon. In terms of sharpness I have not noticed any difference between it and the Nikon 70-200 even though test charts and reviews online say it is not as sharp in the corners, in my opinion from using both it is sharper than the 80-200. I let my friend who has both the 80-200 and the 70-200 have a play with the Sigma and he agrees.
No doubt that there is likely poor copies of the Sigma around as you would expect and this in no doubt accounts for some of the reviews etc. online. My copy is perfect, if it had not of been I would of sold it and got the Nikon.
I let TonyNI on here have a play with it as he was having the same problems making a decision as I was and he went straight out and bought one as well.