I don't really like them but it does make you very noticeable. If they blink a couple times thats alright but the ones that blink the whole time you holding in the brake are kinda annoying...
Saw them for first time in Tennessee. Caught my attention real quick. Have it on my ride now. Been asked many times about it, so it does bring attention to you.
Different states have different rules. In many states, the headlight modulator is only legal during daylight hours. You can buy them with a built in light sensor for that very purpose. Mine works during daylight and stays solid at night. In NJ the brake light flasher is legal day and night.