C1 is the most durable as you will only have to apply once every 2 years according to the blurb! I applied it about a year and a half ago on my Focus and the beading is slowly dropping off now...

It definitely is a use and forget type of product...I actually use the complete G-technique range - C4 for plastic trim, C1 for paint and C5 for rims (although I am told its the same product by G-techniq themselves!). There is a new version of C1 coming out apparently which has a longer work time befor it crystalises.

Be sure to work on say half a panel at a time as if the products crystallise, the only way to get it off is machine polish it!

Other products I use are:

Duragloss 101 I think it is (AWESOME shampoo!)
G Techniq P1 polish
Zaino Z5 PRO polish
Dodo Juice Blue Velvet wax
G techniq C1 for paint
G techniq C4 for trim
G techniq C5 for rims
Rain-X (Durability aint the best but I can generally get around 1-2months out of one application....but the price of it beat everything else!)
Megs window cleaner
AG HD Wax
AG Tyre Shine
Aerospace R303 rubber/plastic dressing
FinisKare detailer
Bilberry wheel cleaner

Cant remember what else lol

The above is what I have narrowed my 'ultimate detailing' kit to...there will be more products I will try and see if they make the cut...but im waiting for the days to get better lol