Having used this lens for a long while i can say it is worth the money. I have produced some superb shots with it attached to my Nikon D3x. well build quality its great if someone gets in your way when ur trying to get a good shot you just bop them on the head with this thing and ur problems solved.
Replying to: Should I Invest In A New Telephoto Lens?
Answer: No, Not this one.
(having never even seen the above items in real life i wouldnt belive anything ive said in my post, accept my answer being no

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First invest in a 200mm prime if you really want to get a serious prime telephoto, skipping to 800mm first time i think would just be a waste of money, and youll end up feeling 'i cant be bothered' with this lens because of its size.
hope this helped