I prefer to filll the comment card while on board and hand it to the corresponding FA; by this means the FA can add the personal ID number to the card and forward it in order to append to their file. Furthermore, the great service is acknowledged right away rather than (somewhat anonymously) afterwards.

Reminds me of a FA, some years ago, that hugged me extensively after receiving the positive comment card; she said something like "we need these comments so badly!". Needless to say that I received some looks from the cabin...