Summary: I have not tried the new 2x but I've heard the 1.7 was the best overall lens and I can say it works perfect for me. no distortion, f stop is still fine for any thing outdoors or well lit indoors. I can't tell I have it on when it comes to size and weight bu I only use it with a 70-200 vr2 nikon lens.Its a big heavy lens itself.The 1.7 is nothing compared the the large lens. with a dx camera I'm getting around 400mm.
Summary: I do event photography, baseball, soccer, football, etc. - I bought this expecting that I might not get a sharp enough image to sell as I usually get from my 70-200 2.8. I was wrong the IQ is fantastic, much better than I expected. Highly recommended if you just want that extra reach now and then but don't want to spring for a 500mm just yet. Buying Nikon costs extra but this one is worth the extra expense.