We use them all the time. Years ago we were ordering them so much I finally got my supplier to stock them. They are great... They say you can put 2- 12/2 nm cables into the cap and I have done it, but it is difficult. Also make sure you have wood around the receptacle as it mounts with screws at ~ 45 ° into the wood.
Get the correct size hole saw- I believe it is 1 3/4", and keep it protected because you want that hole saw to cut perfectly as there isn't much room for error.