Straight mate is bitter. My wife and some people I've shared it with have said the same about it being grassy, I don't taste that myself though. You can also get it with mint or orange or other things if you don't want straight mate (I have these and plain mate so I can pick what I want each day). Some people add sugar or honey to it as well.
EDIT: Also, the first steeping and drink is the most bitter. After that it's much, much less bitter. Comparable to other teas and black coffee.
Ah that is why it is rebrewed. I do a mixed brew in a pair of quart jars. I fill about 2/3 w mate and water. Then I brew oolong tea and mix that strained tea w the mate that still has the mate leaves in it. I drink it w sugar. It's popular for the health and mood benefits it isn't consumed for flavor.
I've been drinking mate since before I could walk and I would disagree. It's an acquired taste but I wouldn't go so far as to say it's 'not consumed for flavor'.
I do something similar to what you do with mint tea instead of oolong for the summertime. A kind of terere cocido I guess lol.
EDIT: Also, the first steeping and drink is the most bitter. After that it's much, much less bitter. Comparable to other teas and black coffee.