I have used a few of their products and they are pretty good, but the sealants on their own don't give the finish a wax does so i tend to use a carnauba wax over the top.
The P1 polish with a wool pad is pretty good on very deep scratches as using it the way Gtechniq describe produces hardly any heat, but it does need a final finish stage afterwards which Gtechniq claim it doesn't, apparently it works by moving the paint around to help remove scratches/swirling
The C1 crystal lacquer leaves a quite a nice gloss but as with all sealants it is a very clinical, cold finish, you need a carnauba wax over the top to give some warmth.
The G1 clear screen kit is about as good as Rain-X to be honest, it lasted about as long as Rain-X but is more expensive as you only get a small bottle of the actual sealant which will just about do one large car.
I haven't used the C5 wheel armour so can't comment but I would imagine it would be the same sort of stuff as the C1 crystal lacquer, which when applied to alloys would form a very strong finish
HTH
Ian