After I assert that I go over it in broad strokes.
Justifies better than saying lots of people still use it. ZK was once the only player in the game and the status quo — regardless of merits, many people will still use it. A good question is: now that there are other options, which direction will people trend towards? Consider how Kafka built consumer groups to not depend on ZK. I’ve also talked directly with the Kafka team about which direction they’re going.
They’re alike consensus wise. Serf is built for service discovery better than ZK is. (You may think: “But ZK’s used for more than service discovery.” — Yes, a good reason for using tools that solve specific problems.) For more details: comparison between Serf and ZK, details on Serf’s internals.