The system I have recent experience of is at best unreliable and at worst downright useless and distracting.
Its ability to detect even the largest 275kv pylon lines is random. I'd say it alerts to less than 50% of what it's supposed to and there is no particular pattern to which ones it will and will not alert. It also regularly generates false warnings.
Plus the system itself spends more time unserviceable than serviceable.
This particular system is negligibly effective in terms of wire strike prevention and almost certainly counter productive to safety in the wider sense (especially when you consider that the antennae regularly fall off the machine!).
Other better systems may be available.