When should you actually start using a service mesh?

hey folks, been thinking about our microservices architecture lately. we're hitting a point where managing inter-service communication, observability, and security is getting pretty complex. weve been handling a lot of this with custom code in each service or using libraries, but its becoming a burden. has anyone here successfully introduced a service mesh like Istio or Linkerd? what were the key indicators that pushed you to adopt one, and what were the biggest challenges you faced during adoption and day-to-day operation? looking for real-world experiences on when its truly worth the overhead.