I was already getting incredibly good battery life on 9..... I'm not sure how it could get much better without starting to affect the way my phone works. So far 10 seems to be doing just as good as 9. I wonder if some folks did a factory reset after installing 10, that alone will increase battery life on any phone that hasn't been reset in awhile, and has had third party apps installed on it.
And by the same token, a phone that has been in use for quite awhile with apps installed on it and no recent factory reset, might not show any battery improvement when upgraded to 10, and maybe even worse battery life depending. My phone was recently factory reset, and I haven't installed any apps on it, and the battery life on 9 was stellar, so I'm not so sure any improvement on 10 would be that noticeable for me.
Every situations going to be different. For maximum battery life I would recommend a factory reset after upgrading to 10 if you haven't done one recently ;-)