We have a winner! Thanks, toxi.
Can't believe I missed this thread. My wife got this for me as our housewarming present. It's amazing. I drink a ton of seltzer so this means I don't have to run to the store all the time just to waste seltzer bottles. 60+ liters per cartridge also seems like it's an accurate statement for me and I get that doing 4 "buzzes" on many of my liters. I'm not really sure if this is saving me much money mostly because I end up drinking more seltzer but it's definitely more convenient both in always having it on hand and not having to deal with empty bottles. My only gripe is my gripe with all seltzer. If you pour it into a cup it goes flat fast. I often drink from the bottle, which--since they're not dishwasher safe--is a minor pain, but still well worth it.
As for flavoring. The cola is okay, mediocre. The diet cola is good, the orange soda is great. The energy drink will make you puke. But, you can always get your own soda syrup from somewhere else or squeeze a lemon or orange into it. Plenty of options.
Also, for BV, QV and South Philly people. Fante's in the Italian market carried refill cartridges. They're the standard $14.99 price with exchange. I was worried I'd have to deal with the hassle of sending it in or driving out to Bed Bath and Beyond but it's right there near me. Fante's also sells some of the official flavorings. There are also a few locations in Center City.