Plain MSG absolutely does not taste bitter. I just tried some (again) to confirm, it's not salty & not bitter. Just a strong flavor of its own.
Almost tastes like fat more than anything.
Probably because a lot of fat sources have high levels of glutamates in them. You're not tasting the fat, per-say, but the other stuff that isn't fat. It's why beef tallow is so much tastier than neutral oil. Same level of fat.