I'm a big fan of Cool Cartos, but I have recently discovered that wow cartos work well with certain flavors like RazzWow. What I find impressive is the longevity of both cartos. I can put 90 ml through a cart, wash it every 30 ml or so before tossing them. The thought of 90 refills, including topping off along the way is crazy and brings the cost of cartos alone down to mere pennies.
When they get tossed it's almost always because they become harder to draw off of, which has never made any sense to me but it happens.