"excellent , very pleased with that item that allows you to add more shelves in the future (the holes are already pre-made so if you can move your shelves at the level that you want, or add more !) very easy to install (took me a minute to set everything), no screwing involved if you don't want to, so you don't actually mess with the furniture. The thing I like is that the shelves is a little less deeper than the kallax case, so if like me you put a plank on the back of the kallax to close it, there is still room to put the cables of your electronic devices behind the shelves with no risk of crushing the cables (game console / internet box etc).
only downsides are :
the delivery fee which I found quite high for a delivery from Germany to France, and the finishing white paint that I would have liked more "opaque", more white !"
Kallax insert with 2 shelves, shelf divider, variably adjustable