I went for the (what I assumed!) were factory fit ones.
I've had a bad experience of those aftermarket BLEEP BL*%DY BLEEP ones.
Guess what... the Nissan ones are just that! they dangle from a flap of cheep lookin flimsy plastic below the bumper and are totally un adjustable so I hate them. I've read some guys on here talking about moving sensors to in bumper which I'm gonna do and i've put insulating tap over the buzzer to quieten it down a bit.
I'm sure you can get better aftermarket kits than Nissans option to summarise!
I bought these: http://cgi.ebay.co.uk/LED-Display-4-Sen ... dZViewItem
been fitted about six weeks now,no problems.I had my steel bumper drilled out at the local blacksmiths,as the supplied holecutter is only fit for plastic bumpers,this stage is critical make sure you get it right first time.The display is located in the dash where I removed my cuphplder from.Let me know if you would like to see pics.