It's faster, but I don't know about calling it a worthwhile upgrade FOR ME. The 1080ti should be a worthwhile upgrade FOR ME.
FTFY
I don't understand why people say blanket statements like this. "Worthwhile" is such a subjective term to use. Everyone has their own interpretation of worth. A better statement would be performance per dollar, which gives potential buyers some gauge to put a value on the product.
I'm looking at the 1070 as an upgrade to my 760ti. Even though it'll push more than 2x the FPS, it's still going to be a hard sell for me. I don't NEED the 1070 to play games but if I can score one for about what the current 970's are going for then, to me, that's possibly a worthwhile upgrade. To my friend, who's budget conscious but wants to squeeze out as much performance as possible whenever he can (on whatever budget he decides at the time), the move from 760 to 970 was worthwhile and now the 970 to 1070 is as well.
Also, it's fluid, moving target. At MSRP a specific upgrade may or may not be worthwhile... but throw in a free game that one might be interested in anyway and the value proposition changes for that person. To someone else, in the same scenario, that's not interested in the game, nothing changes.
I need my coffee. I didn't intend to go on a rant this AM, haha