when do you actually need a circuit breaker pattern

I've been working on a distributed system, and some services talk to each other over the network. We've run into problems where one slow service can cause a chain reaction, taking down other services that rely on it. I've heard that the circuit breaker pattern can help with this, but I'm wondering if it's always necessary or if just retrying the call might be enough sometimes. What's your experience with putting circuit breakers in place? How do you know when it's the right time to use this pattern?