So, after 600K miles on United, I finally took my first flight on a 757-300! (Which means, incidentally, that I've now flown every currently operational kind of UA metal except the 789... and the 3-4-3 777, if you count that as a type.)
Anyway, having selected 1A on the basis of this thread, I want to happily confirm that this is not only the best seat on the plane, but IMO the best domestic F seat in the entire fleet. (Not counting dedicated Hawaii configs).
Why? Three reasons:
1. You get decent legroom even without the cutout. But the cutout is [i]ridiculously[i] big -- big enough to stash a sizeable carryon, and simultaneously stretch your feet out completely. It's about 18" deep, which is hard to believe until you've actually experienced it. The net result is that you basically get unlimited legroom and stowage at your feet.
2. The 753 F seat has good recline -- as good as any other domestic seat I've experienced -- and in addition, the base slides forward a bit. Net result: a napping position that I think is better than any other domestic seat, and almost as good as the old HI 777 (barcalounger) config.
3. For those who care, you are absurdly far forward of the engines (thanks to the planform of the 753). Meaning that you can barely even see them, and have a fantastic view of everything front and back out the window.
Score!