The Tamron looks awesome and is certainly a great deal.... I am honestly tempted!
But I did a bit of a test last night with my currently set-up (70-200 + 2xtc). I do most of my wildlife photography (for fun) in the dusk/dawn time, and f/8 for good IQ would not cut it... hell, f/5.6 is a struggle. I find i use my 70-200 native at 2.8 a lot because the of the speed.
Weather sealing and the build quality of the lens are also important as it is India; it is very dusty and when it rains, IT RAINS!
I would choose the Tamron if most of my wildlife was taking in good light, but it is not! I can't afford the 300 right now, but i am saving, and for me I think it is worth the wait!
Here is an example... a shot taken at 200mm f/2.8 iso 1600 1/200. some extra reach at 2.8 would have been nice! Yes i know i can stop down and bunk the ISO, but i like the choice not to. I am also a shaky person, so crazy low shutter speeds kill me with still subject let alone a moving one!
ps - I would like to thank everyone on here for their in-hands review of this lens and comparison to the 300 combo. It has really helped me make up my mind... I just need to sway the wife now!