I've also got the Thaiss version of Isotop (cheaper and good quality) and have to say I use it more than the outside topper. The reason is that I fit it when I'm already on bed upstairs, and beging feeling chilli (or frozen). It's very easy to un/install, so I just have it always handy.
I've used to use a lot the external one when I was a newbie, but one all fears are gone, I use it when it's particulary windy, rainy or it's very cold outside (snow)... and I prettt sure most of the times I could spare it.
@simba, it fits properly and the velcro (only in awning side) is strong enought. Thd only thing I've improved is because with the awning there is not much rail available to secure the top, being a little anoying when very windy. I'm using now some soecial pegs I've got in Clash Olson to secure it firmily.... not perfect yet, but good enougth.
It can be attached to the windscreen and windows protector to make the set more reliable.
@SimonB, mine one is a Cali Beach, so no front window, but you can order with it. Actually you can ask for an ad-hoc transparent windos in the roof if you have solar panels.
The material is very good, waterproof and breathable. Not sure whether it fit in thail gate, I think so, but need to try to say.
In general the quality is pretty good despite some design issues you can manage easily.
As any European product it have to have two years of warranty but, it's not made in UK (Assuming your Cali is German and you're browsing other German options, I guess that is not a MUST but a WHISH). The fabric I think is made by Dupont in UK.