I never heard of it... until last week, qhen my boss told me to play.
I know, it's kind of surreal. xD
Anyway, we played and without notice, we do it for most than an hour. It's a classical game bassed on particles and mass, but in his simple gameplay lay the fun.
I also liked the option to join or create game-rooms to play with friends and teams.
It's better that you try it by yourself, don't rely only on a Youtuber opinion (neither a forum user like me, hehe).
It's a nice game to play while you're in a Queue for another game, or waiting on something.
I also like their snake game. I believe it was slither.io
Though that can be a bit more frustrating and competative than Agar.io
It wouldn't surprise me if they've made more "IO" games.
Definately think Agar is much more peaceful to play than Slither though, but I suggest if you want more of a challenge to play slither.io since you can dash forward as the snake and cut other people off, then devour their delicious souls to grow bigger.
I haven't played it since it has been released. ACTUALLY there is no fun any more... There are just teams. I was a pro player who hit 50000 points once I destroyed a huge team just my myself. I wouldn't play such games any more.
Use to play it but team killing is a major issue. It got old slowly. Sither.io was my next addiction but that had a tone of lagg and the smaller snakes would rush you and wipe you out because of the lagg.