Service Mesh: When should you get one, and when is it just too much?

I've been checking out service meshes like Istio and Linkerd lately. They seem to offer a lot for traffic control, seeing what's happening, and keeping things secure with microservices. But then again, it also feels like a huge amount of extra work just to add another layer. For smaller teams or apps with just a few services, is it truly worth it? When do you guys actually start thinking about a service mesh, maybe based on how many services you have or how complicated your traffic gets? Or are there certain situations where it's a definite must, even for simpler setups?