I have one of the Keg King carbonation caps - they do a great job, one of the dual style ones that accept both gas / liquid. They are super handy to get 1/2 a tallie of beer from the fermenter, chill in the freezer for a while in an PET bottle then force carbonate to test the quality of the beer before you keg / bottle it.
In saying that, I really want one of those CPBF's mentioned in the first post, you can actually use them as a pass-through and attach a tap on the end. Be a little quicker then disconnecting everything and putting the carbonation cap on as it is always connected. This is the tap setup from All-Inn Brewing that they have posted on their facebook page, Ross told me the other week to checkout their setup when I was asking him about that filler.