I'm sure that my total enjoyment of Halo 5 will be based on how the main menu compares to other Halo games.
Perhaps not if you're just going to boot it up and queue into matchmaking. For people who spend more time in the menu editing Forge objects, organizing Custom Games and searching for files, the user interface is a larger presence. Halo nearly perfected this process with Halo 3 and Reach and then took a steep dive downward.
UI ties the gameplay together and it is just as important as the game's story, mechanics, level design, or whatever else. Obviously the game as a whole is going to dip in quality if any one of those things are lacking. Nevertheless, the aforementioned games being "shit" doesn't take away from what they did right.
If Halo 5's UI is sloppy, unresponsive and unable to provide basic functionality that Halo has in the past, it's going to sour the overall package. Your total enjoyment would probably go down if you had to constantly scroll through submenus and wait for them to load. 343's passed efforts in UI don't inspire much confidence, so skepticism is fair, especially when the bar was set as high as it was.
I enjoy the Xbox One a whole lot less than the 360 almost exclusively because of the terrible user interface. Playing any game on a console that may as well exist in Cobb's mind makes the entire experience less enjoyable the moment I press the guide button. Halo 5 absolutely deserves to have state of the art font, icons and scrollbars embedded into every inch of the game. I'm running out of patience for anything less.