Interestingly, I get the new site in Chrome (most of the time) or Opera (all of the time), but always the old site on IE/Edge.
Frustratingly, when using the new site to look for award travel for next summer, the new site shows flights that do not show up on the old site. But I cannot book any flights through the new site and calling United, several of the flights that the new website shows me don't even show up on the CSR's computer.
So, the new site is pretty - but it needs to show real flights and actually let me book them to be in any way useful.