The licensing cost of Zing generally makes this a bad trade-off. It's much cheaper to purchase more hardware. Zing is targeted at vertically scaling very large JVM heaps, where it's valuable to have massive amounts of data on a single, big machine.

As an ex-Azul employee I can say there's a good number of Azul clients using a Zing+Cassandra setup, so the price point is right for some people at the very least. Zing licence cost has also changed in recent years (3.5k per server last I looked, and that is before you haggle some bulk deal) so not sure if your impression is calibrated to that new price point.

Have friends who have used it, they report that it works reasonably well. Especially in p99.

By what factor/magnitude p99 was improved ? Any idea ?

Actually, it appears that is one of the premises[1] they sell Zing on.

[1]: https://www.azul.com/solutions/cassandra/