Coatings like the Williams are probably a good idea if you haven't got time or can't be bothered with waxing etc but if you get a kick out of keeping the car shiny I'm not sure they're worth the expense.
I used to get a good result on my last Z with the coating but I could never tell how much that was down to a decent wax and how much part the coating played.
I use Jetseal 109 on my current Z - lasts for months and costs a lot less than the Williams type stuff (£30 compared to £300+) (plus you can remove and re-coat a few times a year, and add a layer of wax every now and then to boost the shine).
Additionally, the Wiliams definitely didn't prevent those occasional little hairline scratches and little surface scuffs that every car seems to pick up from washing etc. The easy way to remove them is a quick polish with a cloth and something like Autoglym Super Resin polish, but presumably you remove a patch of the coating at the same time.