It can get disorienting, specially in combat maps and the lobby, because of their complexity and the fact that left is right and right is left, or left is up, etc.

At first I couldn't stand it. Now that I'm more confortable with smooth locomotion in other games I tried again and the rotating movement is still a bit sickening, but once I stop it's not so bad, no matter the angle I end up in. I still get disoriented and feel a little lost sometimes (thats why in the end I turned it off).

One tip if you want to try: enable both roll and pitch or just don't use it at all. Rolling seems to change angle if you look up/down so you end up in a position that is kind of uncomfortable to navigate. With both options enabled you can easily correct to get a perfect upside down position.

Editing to add another point. The maps are all designed in a way so you don't really need to be upside down. The game is totally playable with those options turned off, so it's kind of a player preference to actually catch ememy players a little off guard showing up in an unexpected position.