Is it worth buying some for application to a brand new car? I have always used diamond brite but things move on.
It depends how you're using your cars, what you want the coating to achieve for you and your budget. I'd always argue that PPF is better than CC for stone chip protection based on my experience.
Some of my cars have Xpel PPF and others have ceramic coating. I'd recommend a detailer accredited application versus picking up a consumer product. I don't think consumer products are as durable and with the professional grade products, they're so hard when cured, to remove them, you have to wet sand the whole car so it's not something that you want to get wrong which is why manufacturers insist on certification and strictly limit sales of the product to professionals only.
Preparation is key and having applied a consumer product once, you have to be so meticulous to avoid trapping any imperfections. Detailer studios generally have much better dust prevention and management than your own garage.
I had the option of Kamikaze Enrei versus Gtechniq but elected Gtechniq Crystal Serum Ultra as the warranty was 9 years versus 5 and costs were not far apart. I rationalised that in the event that I sell the car, any time remaining on the warranty would be of greater appeal to a buyer.
With both, you get a registration plate and paperwork from an accredited detailer. I would check that the detailer is accredited to ensure you have protection before giving them the business.