Immutability is absolutely fantastic as a default (especially for typical business logic). I'm not sure I want to go back to mutability except in constrained environments or time-sensitive software. Mutability should be harder to reach for because it forces you to think hard about the implications to other parts of your program. I can think of a number of times we've had to parallelize a routine or share resources between workers, and it has been almost trivial _because_ of immutability. In contrast to that, I inherited a hairball of C code using pthreads, mutexes, global variables, etc which had to be completely gutted and rewritten as it was impossible to understand the dependencies in the software (we were witnessing segfaults, deadlock, etc).
