Very good. Quality after-market windows for VW T5 vans are only produced by a handful of decent companies, while lower-grade, cheaper versions are manufactured by a few Far East factories. You can tell the cheap ones by the thin, wobbly glass and they may also be missing the important CE/TUV stamp. We started importing these windows a few years ago and have had very few issues with quality.
We now source the barn door windows from an OEM factory as the Turkish and Chinese manufacturers seem to have trouble getting the curvature on these particular windows quite right