Iv had these type (not this brand though) of systems fitted to a good number of customers cars and in my experience they are a bit hit and miss. If they work they work well but if they don't they are rubbish. I don't sell these type of systems anymore as they turned out to be more trouble than they were worth. This is all in my experience of course and to be fair havnt come across hawk systems so may be worth a punt for £20.
I didn't want sensors drilled into my bumper as I prefer the clean look so I ended up with a reverse camera which kinda does the same thing