I’ve used the normal non-zip k-pact knee pads for a fair few years now and they’re great and saved my knee’s on multiple occasions.
Did look at the zip ones and tried a pair, but found them less comfortable than the non zip versions and just couldn’t see myself taking them on & off during rides – likelihood is I take them off, not be arsed to put them back on for ‘this short downhill’ and then crash! The normal ones are plenty comfortable to pedal in for a fair few hours even in the heat.
The “D3O” inserts are made by SAS-Tec who’s main market is making the inserts for motorcycle leathers, so they give great protection. Find their miracle foam stuff lighter, more comfortable and more mailable when cool than D3O and some others I’ve tried