James,
I have used this on both of my Maserati's over the last 5 years, over that time I used two tubs of the stuff; and I waxed every second wash, and washed the car after every use.
I was sold on the brand after having my 1st 4200 detailed by Clifton Care Care in Bristol as the car came up like...no, better than new. It does not contain any abrasivies, smells devine, does not leave any residue and hardens after use if left in the bright sun...most of all you do not get any swirl marks on dark cars, perfect for black like yours.
I used the mid range wax at about £65 per tub [Onyx] and my friends noted just how well water beaded off the car and how a hose would blast off most dirt without the need to wipe.
I would suggest gooing for one of the sets for about £150...after all thats just 15 washes at my local Albanian Car Wash and I think they use drain water and tesco washing up liquid.
They also do a tester kit...ideal for one wing, only a fiver.
http://www.swissvax.co.uk/product_detail/wax_products/tester_kit_with_onyx_wax_2