I'm finally finished applying the various Gtechniq products to protect my new car so here are my final thoughts about the experience.
I don't want to apply C1 again. There. I said it. I'm confident that C1 will provide the protection that I want. The problem for me is: it's quite difficult to apply well and mistakes are very costly. A professional detailer with experience using these products is probably much more skilled and works in a very well lit bay, so they are much less likely to make mistakes. Having said that, I don't think it's worth paying them over $300 to apply C1.
I enjoyed applying EXOv2. I'll be buying more in about a year to reapply over the C1 because Gtechniq says EXOv2 lasts 12-18 months and C1 lasts much longer. After the C1 wears off, I may use EXOv2 again without the C1. I'm wondering if it can be applied on top of wax (after the wax has cured, of course). In the past I've used Zaino CS in that way to get a nice long lasting wet look. Update: Rob Earle of Gtechniq has informed me that EXOv2 bonds to the layer below it (paint or C1 etc) and so applying it atop wax will not have the same effect.
I'm a little concerned about the effects L1 had on my cotton T-shirt applicator. So I'm waiting and watching to see how it wears on the leather.
I'm not impressed with G5. It leaves marks from any touch after application, even days later. Weeks later I was still buffing the glass.
This was relatively easy to apply. I'll hold my judgement until I see how it performs and how long it lasts, but I wouldn't dread applying it again.
G4 worked as advertised. It's a very fine polish for glass that's relatively easy to use. I plan to use it again as the glass on my car gets dirty and abused over the coming years.