I use my nines to "hide" some beer I want to let age (as I normally drink the first half from cornies, then hide) I can put 2 nines on the hump of me freezer, I use the others for parties, line cleaners, etc..
Bulk buy? You're not going to get them any cheaper than $80ea from eBay Germany.
I think they're great not just for taking to parties, away camping etc. but for splitting batches for aging, kegging small experimental batches, or occasionally swapping full keg for full keg with your other brewing mates.
9L kegs are awesome. I couldn't do without them!
*batches where you want to bottle half, keg half
*batches where you want to age half, & drink half straight away
*when you don't have enough fridge space
*can transfer from 19L kegs when they're half empty to free up more fridge space
*they look cool