We've had Taylor Made external screens for about 6 years and they still work well (though a bit dirty now). I keep looking around because advances since mean that they are now less bulky. It's worth paying for something like Silver Screens or Taylor Made though as the cheaper ones look far less efficient.
Just bought silver screens for our new van, fantastic service, good fit, fold down pretty flat for storage and very effective. We have a PVC and it was a chilly with a light frost and we had kept the heating on low overnight to test it and not a spot of condensation. Very pleased.