Just fine is pushing it, we are capable but not particularly good.
We need specific plant parts grown on purpose to be consumable (fruit, nuts, ...), or we need cooking.
There is a good number of plant stuff like grass, certain bark, and most parts of medium sized plants which a lot of animals can consume but we cannot.
If you go out and eat random plants ~~you will die~~ you won't be able to digest almost any of that.

I would argue there are loosely two levels of plant digestion ability above ours.
The first being what most mamals have, which allows to consume leafs and most small and medium plants minus the thicker stalks. My example would be deer.
Then there are ruminants who can digest grass more efficiently and tend to deal with stalks better, main difference being more things being worth digesting vs. just being digestible. Classic example is cows.