My other half reported an incident recently (after the two of us having the 'is it worth it' conversation). **Positive experience.** Taken seriously. It may have helped that the person had stopped their car ahead, and reversed a couple of times to threaten, and that there was a witness. Didn't result in anything (no number plate recorded) but they went as far as checking nearby CCTV (no success).
I'd echo comments above. Might be a waste of time, might be worth it, but gets stuff recorded. Be open to disappointment. Be aware of the risk of prejudice (My personal suggestion would be to dress in ordinary clothes, don't be 'a cyclist', don't be angry, just be an ordinary human who has been scared to death). Police officers are human and subject to all the normal prejudices, assumptions, stereotypes, etc etc. Not how it should be, but there you go...