I started toying with mattermost as well and then saw in the logs things going to "matteranalytics" or something like that... Shouldn't be too surprised, but seemed ironic since I looked it up to avoid the similar thing with rocket
I've set up a test server for matrix as well, wanted to compare to rocket since I see it mentioned a lot.
When I told a friend (the one I'd need on board to get the group of friends to use it) he said "does it have the gifs?" Matrix has a years-long enhancement request for native giphy support which is yet resolved. I dabbled with a giphy plugin with matrix and it worked alright, but I dunno if it was good enough to win my friends over.
Im amused that the repo for it is on github and not on, well, Gitness